Label Element Value
Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName Legg Mason Global Asset Management Variable Trust
Prospectus Date rr_ProspectusDate Feb. 28, 2014
Class I | Legg Mason BW Absolute Return Opportunities VIT
  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading LEGG MASON
BW ABSOLUTE RETURN OPPORTUNITIES VIT
Objective [Heading] rr_ObjectiveHeading Investment objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock Positive returns that are independent of market cycles.
Expense [Heading] rr_ExpenseHeading Fees and expenses of the fund
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock The accompanying table describes the fees and expenses that you may pay if you buy and hold Class I shares of the fund. The fee table and expense example do not reflect expenses incurred from investing through a separate account or qualified plan and do not reflect variable annuity or life insurance contract charges. If they did, the overall fees and expenses would be higher than those shown. Detailed information about the cost of investing in this fund through a separate account or qualified plan is presented in the contract prospectus through which the fund’s shares are offered to you or in the information provided by your plan.
Shareholder Fees Caption [Text] rr_ShareholderFeesCaption Shareholder fees
(fees paid directly from your investment)
Operating Expenses Caption [Text] rr_OperatingExpensesCaption Annual fund operating expenses (%)
(expenses that you pay each year as a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination December 31, 2015
Portfolio Turnover [Heading] rr_PortfolioTurnoverHeading Portfolio turnover.
Portfolio Turnover [Text Block] rr_PortfolioTurnoverTextBlock The fund pays transaction costs, such as commissions, when it buys and sells securities (or "turns over" its portfolio). A higher portfolio turnover rate may indicate higher transaction costs. These costs, which are not reflected in annual fund operating expenses or in the example, affect the fund's performance. The fund is newly offered; therefore, it does not have a turnover rate to report for the most recent fiscal year.
Other Expenses, New Fund, Based on Estimates [Text] rr_OtherExpensesNewFundBasedOnEstimates “Other expenses” are estimated for the current fiscal year. Actual expenses may differ from estimates.
Expense Example [Heading] rr_ExpenseExampleHeading Example
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock This example is intended to help you compare the cost of investing in Class I shares of the fund with the cost of investing in other mutual funds. The example does not include expenses incurred from investing through a separate account or qualified plan. If the example included these expenses, the figures shown would be higher. The example assumes:
  • You invest $10,000 in the fund for the time periods indicated
  • Your investment has a 5% return each year and the fund’s operating expenses remain the same
  • You reinvest all distributions and dividends without a sales charge
Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example by, Year, Caption [Text] rr_ExpenseExampleByYearCaption Number of years you own your shares ($)
Expense Example, No Redemption, By Year, Caption [Text] rr_ExpenseExampleNoRedemptionByYearCaption Number of years you own your shares ($)
Strategy [Heading] rr_StrategyHeading Principal investment strategies
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock Under normal market conditions, the fund seeks to meet its investment objective through strategic investments in domestic and foreign fixed income securities and currencies. The fund will initiate long and short exposures by investing across bond sectors, including sovereign debt and corporate bonds; currencies; and derivative instruments. Short exposure will be established primarily through the use of derivatives, including currency forwards, interest rate swaps and futures.

The fund typically has significant exposure to foreign currencies and foreign countries. The fund may invest a significant portion of its assets in a particular geographic region or country, including emerging market countries. Normally, the fund will not invest more than 20% of the fund’s assets in securities or issuers in any one foreign country, other than the United States, Canada, the United Kingdom, Japan, Australia and member countries of the European Union, or denominated in any one currency, other than the U.S. dollar, the Canadian dollar, the pound, the euro, the Australian dollar or the yen.

The fund may invest in the debt and currencies of sovereign nations, as well as derivative instruments and fixed income securities that offer exposure to interest rates of sovereign nations. The fund may also invest in corporate bonds and other fixed income securities. While the fund may invest in securities of any rating level, or unrated securities, the portfolio managers intend to maintain an average weighted portfolio quality of A- or better, whether composed of rated securities or unrated securities deemed by the subadviser to be of comparable quality. The fund will invest in both investment grade and below investment grade securities and currencies of developed and emerging countries. Fixed income securities rated below investment grade are commonly known as “junk bonds.” Investment grade securities are securities rated at the time of purchase by at least one nationally recognized statistical ratings organization (“NRSRO”) within one of the top four categories, or, if unrated, judged by the adviser to be of comparable credit quality. Fixed income securities in which the fund may invest are debt securities issued or guaranteed by national governments, their agencies or instrumentalities and political sub-divisions (including inflation index linked securities and municipal bonds); debt securities of supranational organizations such as bonds, debentures and freely transferable promissory notes; corporate debt securities, including debentures, bonds (including zero coupon bonds), convertible and non-convertible notes, commercial paper, certificates of deposits, freely transferable promissory notes and bankers acceptances issued by industrial, utility, finance, commercial banking or bank holding company organizations; mortgage-backed securities (including collateralized debt obligations); asset-backed securities; emerging markets debt; and high yield debt (often called “junk bonds”).

The fund may invest in securities of any maturity. The maturity of a fixed income security is a measure of the time remaining until the final payment on the security is due. The fund’s investments may be of any duration. The weighted average effective duration of the fund’s portfolio, including derivatives, however, is expected to range from -5 to +5 years. Duration is a measure of the underlying portfolio’s price sensitivity to changes in prevailing interest rates. The fund may have a negative weighted average effective duration due to holding certain instruments that have negative effective duration, such as through the use of derivative instruments.

The fund achieves certain investment exposures, including short positions, primarily through derivative transactions, including foreign currency forwards, bond futures, interest rate futures, swaps (including interest rate, total return and inflation swaps), credit default swaps, credit default swap index products, instruments involved in currency risk management strategies, including cross hedges, options and options on futures and warrants. The fund may use derivatives to enhance total return, to hedge against fluctuations in securities prices, interest rates or currency exchange rates, to change the effective duration of its portfolio, to manage certain investment risks and/or as a substitute for the purchase or sale of securities or currencies. The fund’s use of derivatives may be extensive. However, the notional principal of the fund’s aggregate net short currency exposure will not be lower than -70% of the fund’s total assets. When the fund enters into derivative transactions, it may be required to segregate assets, or enter into offsetting positions, in accordance with applicable regulations.

The fund is classified as “non-diversified,” which means it may invest a larger percentage of its assets in a smaller number of issuers than a diversified fund.
Risk [Heading] rr_RiskHeading Principal risks
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock Risk is inherent in all investing. There is no assurance that the fund will meet its investment objective. The value of your investment in the fund, as well as the amount of return you receive on your investment, may fluctuate significantly. You may lose part or all of your investment in the fund or your investment may not perform as well as other similar investments. The fund may take temporary defensive positions; in such a case, the fund will not be pursuing its principal investment strategies. The following is a summary description of certain risks of investing in the fund.

Market and interest rate risk. The market prices of the fund’s securities may go up or down, sometimes rapidly or unpredictably, due to general market conditions, such as real or perceived adverse economic or political conditions, inflation, changes in interest rates, lack of liquidity in the bond markets or adverse investor sentiment. When market prices fall, the value of your investment will go down. The value of your investment will generally go down when interest rates rise. A rise in rates tends to have a greater impact on the prices of longer term or duration securities.

Issuer risk. The value of a security can go up or down more than the market as a whole and can perform differently from the value of the market as a whole, often due to disappointing earnings reports by the issuer, unsuccessful products or services, loss of major customers, major litigation against the issuer or changes in government regulations affecting the issuer or the competitive environment. The fund may experience a substantial or complete loss on an individual security.

Non-diversification risk. The fund is classified as “non-diversified,” which means it may invest a larger percentage of its assets in a smaller number of issuers than a diversified fund. To the extent the fund invests its assets in a smaller number of issuers, the fund will be more susceptible to negative events affecting those issuers than a diversified fund.

Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield, value or market trends affecting a particular security, industry, sector or region, or about market movements is incorrect.

Model risk. The subadviser’s investment models may not adequately take into account certain factors and may result in the fund having a lower return than if the fund were managed using another model or investment strategy.

Market sector risk. The fund may be significantly overweight or underweight certain companies, industries or market sectors, which may cause the fund’s performance to be more sensitive to developments affecting those companies, industries or sectors.

Absolute return strategy risk. The fund uses a variety of investment strategies, sometimes referred to as absolute return strategies, to try to preserve capital while maximizing absolute returns. The portfolio managers do not attempt to keep the portfolio structure or the fund’s performance consistent with any designated stock, bond or market index, and during times of market rallies, the fund may not perform as well as other funds that seek to outperform an index. Over time, the investment performance of absolute return strategies has typically been substantially independent of longer term movements in the stock and bond markets. Interest rate levels and currency valuations will not always respond as the portfolio managers expect and portfolio securities may remain over- or under-valued. Because a significant portion of the fund’s assets may be invested in a particular geographic region or country, the value of fund shares may be affected by events that adversely affect that region or country and may fluctuate more than that of a fund that is not so invested.

Long/short strategy risk. While the portfolio managers may invest in long positions and short positions, there is the risk that the investments will not perform as expected and losses on one type of position could more than offset gains on the other, or the fund could lose money on both positions, if the portfolio managers judge the market incorrectly.

Liquidity risk. Some assets held by the fund may be impossible or difficult to sell, particularly during times of market turmoil. These illiquid assets may also be difficult to value. If the fund is forced to sell an illiquid asset to meet redemption requests or other cash needs, the fund may be forced to sell at a loss.

Leveraging risk. The value of your investment may be more volatile if the fund borrows or uses derivatives or other investments that have a leveraging effect on the fund’s portfolio. Other risks also will be compounded. This is because leverage generally magnifies the effect of a change in the value of an asset and creates a risk of loss of value on a larger pool of assets than the fund would otherwise have had. The fund may also have to sell assets at inopportune times to satisfy its obligations. The use of leverage is considered to be a speculative investment practice and may result in the loss of a substantial amount, and possibly all, of the fund’s assets.

The subadviser expects that the implementation of the fund’s investment strategy, which may include a significant level of investment in derivatives, could have the effect of creating leverage in the fund in that the fund’s potential exposure may be greater than its net assets.

Credit risk. If an issuer or guarantor of a security held by the fund or a counterparty to a financial contract with the fund defaults or is downgraded, or is perceived to be less creditworthy, or if the value of the assets underlying a security declines, the value of your investment will typically decline. Junk bonds are considered speculative, have a higher risk of default, tend to be less liquid and are more difficult to value than higher grade securities. Junk bonds tend to be volatile and more susceptible to adverse events and negative sentiments. Subordinated securities are more likely to suffer a credit loss than non-subordinated securities of the same issuer and will be disproportionately affected by a default, downgrade or perceived decline in creditworthiness.

Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will not benefit from the rise in market price that normally accompanies a decline in interest rates, and will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security. The fund may also lose any premium it paid on the security.

Mortgage-backed and asset-backed securities risk. When market interest rates increase, the market values of mortgage-backed securities decline. At the same time, however, mortgage refinancings and prepayments slow, which lengthens the effective duration of these securities. As a result, the negative effect of the interest rate increase on the market value of mortgage-backed securities is usually more pronounced than it is for other types of fixed income securities, potentially increasing the volatility of the fund. Conversely, when market interest rates decline, while the value of mortgage-backed securities may increase, the rates of prepayment of the underlying mortgages also tends to increase, which shortens the effective duration of these securities. Mortgage-backed securities are also subject to the risk that underlying borrowers will be unable to meet their obligations and the value of property that secures the mortgage may decline in value and be insufficient, upon foreclosure, to repay the associated loan. Investments in asset-backed securities are subject to similar risks.

Inflation-indexed securities risk. The values of inflation-indexed fixed income securities generally fluctuate in response to changes in real interest rates (approximately nominal interest rates minus the inflation rate). Although the principal value of many inflation-indexed securities declines in periods of deflation, holders at maturity receive no less than the par value of the security. However, if the fund purchases inflation-indexed securities in the secondary market whose principal values have been adjusted upward due to inflation since issuance, the fund may experience a loss if there is a subsequent period of deflation or lower level of inflation. If inflation is lower than expected during the period the fund holds an inflation-indexed security, the fund may earn less on the security than on a conventional bond. Because an increase in principal value of an inflation-indexed security is treated as taxable income to the owner in the year the adjustment is made, even though no cash is paid out, the fund could be required to sell other securities to pay taxes on this income, including when it is not advantageous to do so. Inflation-indexed securities do not protect against the decline in value of debt securities caused by increases in nominal interest rates.

Extension risk. When interest rates rise, repayments of fixed income securities, particularly asset- and mortgage-backed securities, may occur more slowly than anticipated, extending the effective duration of these fixed income securities at below market interest rates and causing their market prices to decline more than they would have declined due to the rise in interest rates alone. This may cause the fund’s share price to be more volatile.

Foreign investments and emerging markets risk. The fund’s investments in securities of foreign issuers or issuers with significant exposure to foreign markets involve additional risk. Foreign countries in which the fund may invest may have markets that are less liquid, less regulated and more volatile than U.S. markets. The value of the fund’s investments may decline because of factors affecting the particular issuer as well as foreign markets and issuers generally, such as unfavorable government actions, and political or financial instability. Lack of information may also affect the value of these securities.

The risks of foreign investments are heightened when investing in issuers in emerging market countries. Emerging market countries tend to have economic, political and legal systems that are less fully developed and are less stable than those of more developed countries. They are often particularly sensitive to market movements because their market prices tend to reflect speculative expectations. Low trading volumes may result in a lack of liquidity and in extreme price volatility.

Currency risk. The value of investments in securities denominated in foreign currencies increases or decreases as the rates of exchange between those currencies and the U.S. dollar change. Currency conversion costs and currency fluctuations could erase investment gains or add to investment losses. Currency exchange rates can be volatile, and are affected by factors such as general economic conditions, the actions of the U.S. and foreign governments or central banks, the imposition of currency controls and speculation.

Sovereign debt risk. Sovereign government and supranational debt involve many of the risks of foreign and emerging markets investments as well as the risk of debt moratorium, repudiation or renegotiation and the fund may be unable to enforce its rights against the issuers.

Derivatives risk. Using derivatives can increase fund losses and reduce opportunities for gains when market prices, interest rates or the derivatives themselves behave in a way not anticipated by the fund. Using derivatives also can have a leveraging effect and increase fund volatility. Certain derivatives have the potential for unlimited loss, regardless of the size of the initial investment. Derivatives may be difficult to sell, unwind or value, and the counterparty may default on its obligations to the fund. The U.S. government is in the process of adopting and implementing regulations governing derivatives markets, including mandatory clearing of certain derivatives, margin and reporting requirements. The ultimate impact of the regulations remains unclear. Additional regulation of derivatives may make them more costly, may limit their availability, may disrupt markets or may otherwise adversely affect their value or performance.

Credit default swap contracts involve heightened risks and may result in losses to the fund. Credit default swaps may be illiquid and difficult to value, and they increase credit risk since the fund has exposure to both the issuer whose credit is the subject of the swap and the counterparty to the swap.

Currency derivatives risk. Currency futures, forwards or options may not always work as intended, and in specific cases the fund may be worse off than if it had not used such instrument(s). There may not always be suitable hedging instruments available. Even where suitable hedging instruments are available, the portfolio managers may determine not to hedge the fund’s currency risks.

Segregated assets risk. In connection with certain transactions that may give rise to future payment obligations, including borrowings and many types of derivatives, the fund may be required to maintain a segregated amount of cash or liquid securities to cover the position. Segregated securities cannot be sold while the position they are covering is outstanding, unless they are replaced with other securities of equal value. As a result, there is the possibility that segregation of a large percentage of the fund’s assets may, in some circumstances, limit the portfolio managers’ flexibility.

Short positions risk. Short positions involve leverage and there is no limit on the amount of loss on a security that is sold short. The fund may suffer significant losses if assets that the fund sells short appreciate rather than depreciate in value. The amount of any gain will be decreased, and the amount of any loss increased, by the amount of the premium, dividends, interest, or expenses the fund may be required to pay in connection with the short position.

Valuation risk. The sales price the fund could receive for any particular portfolio investment may differ from the fund’s valuation of the investment, particularly for securities that trade in thin or volatile markets or that are valued using a fair value methodology. Investors who purchase or redeem fund shares on days when the fund is holding fair-valued securities may receive fewer or more shares or lower or higher redemption proceeds than they would have received if the fund had not fair-valued the security or had used a different valuation methodology.

Recent market events risk. The global financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities and unprecedented volatility in the markets.

Governmental and non-governmental issuers (notably in Europe) have defaulted on, or been forced to restructure their debts; and many other issuers have faced difficulties obtaining credit or refinancing existing obligations. These market conditions may continue, worsen or spread, including in the United States, Europe and elsewhere. Further defaults or restructurings by governments and others of their debt could have additional adverse effects on economies, financial markets and asset valuations around the world. In response to the crisis, the U.S. government and the Federal Reserve, as well as certain foreign governments and their central banks have taken steps to support financial markets, including by keeping interest rates at historically low levels. More recently, the Federal Reserve has reduced its market support activities. Further reduction or withdrawal of this support, failure of efforts in response to the crisis, or investor perception that such efforts are not succeeding could negatively affect financial markets generally as well as result in higher interest rates, increase market volatility and reduce the value and liquidity of certain securities.

This environment could make identifying investment risks and opportunities especially difficult for the subadviser, and whether or not the fund invests in securities of issuers located in or with significant exposure to countries experiencing economic and financial difficulties, the value and liquidity of the fund’s investments may be negatively affected. In addition, policy and legislative changes in the United States and in other countries are affecting many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not bey known for some time.

Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if a fee limitation is changed or terminated or if average net assets decrease. Net assets are more likely to decrease and fund expense ratios are more likely to increase when markets are volatile.

Funds of funds investments risk. The fund may be an investment option for other Legg Mason-advised mutual funds that are managed as “funds of funds.” As a result, from time to time, the fund may experience relatively large redemptions or investments and could be required to sell securities or to invest cash at a time when it is not advantageous to do so.

These risks are discussed in more detail later in this Prospectus or in the SAI.
Risk Lose Money [Text] rr_RiskLoseMoney You may lose part or all of your investment in the fund or your investment may not perform as well as other similar investments.
Risk Nondiversified Status [Text] rr_RiskNondiversifiedStatus Non-diversification risk. The fund is classified as “non-diversified,” which means it may invest a larger percentage of its assets in a smaller number of issuers than a diversified fund. To the extent the fund invests its assets in a smaller number of issuers, the fund will be more susceptible to negative events affecting those issuers than a diversified fund.
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading Performance
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock The fund is newly offered. Once the fund has a performance record of at least one calendar year, the fund’s performance will be included in its Prospectus. The fund will make updated performance information available by calling the fund at 1-877-721-1926.
Performance One Year or Less [Text] rr_PerformanceOneYearOrLess The fund is newly offered. Once the fund has a performance record of at least one calendar year, the fund’s performance will be included in its Prospectus.
Performance Availability Phone [Text] rr_PerformanceAvailabilityPhone 1-877-721-1926
Class I | Legg Mason BW Absolute Return Opportunities VIT | Class I
  
Risk/Return: rr_RiskReturnAbstract  
Maximum sales charge (load) imposed on purchases rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice    [1]
Maximum deferred sales charge (load) rr_MaximumDeferredSalesChargeOverOther    [1]
Management fees rr_ManagementFeesOverAssets 0.65%
Distribution and service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets none
Other expenses rr_OtherExpensesOverAssets 1.33% [2]
Total annual fund operating expenses rr_ExpensesOverAssets 1.98%
Fees Waived and/or expenses reimbursed rr_FeeWaiverOrReimbursementOverAssets (1.23%) [3]
Total annual fund operating expenses after waiving fees and/or Reimbursing expenses rr_NetExpensesOverAssets 0.75%
1 year rr_ExpenseExampleYear01 $ 77
3 years rr_ExpenseExampleYear03 503
1 year rr_ExpenseExampleNoRedemptionYear01 77
3 years rr_ExpenseExampleNoRedemptionYear03 $ 503
[1] N/A
[2] “Other expenses” are estimated for the current fiscal year. Actual expenses may differ from estimates.
[3] The manager has agreed to waive fees and/or reimburse operating expenses (other than interest, brokerage commissions, dividend expense on short sales, taxes, extraordinary expenses and acquired fund fees and expenses) so that total annual operating expenses are not expected to exceed 0.75%, subject to recapture as described below. This arrangement cannot be terminated prior to December 31, 2015 without the Board of Trustees’ (the “Board”) consent. The manager is permitted to recapture amounts waived and/or reimbursed to a class within three years after the fiscal year in which the manager earned the fee or incurred the expense if the class’ total annual operating expenses have fallen to a level below the limits described above.