Summary of Significant Accounting Policies (Financial Assets and Liabilities at Fair Value) (Details) (USD $)
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Cash and Cash Equivalents, Fair Value Disclosure $ 257,236,000   $ 368,531,000
Fair Value, Transfers Between Level 1 and Level 2, Description and Policy [Abstract]      
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount 0 0  
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount 0 0  
Fair Value, Liabilities, Level 1 to Level 2 Transfers, Amount 0 0  
Fair Value, Liabilities, Level 2 to Level 1 Transfers, Amount 0 0  
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Transfers, Net [Abstract]      
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers Into Level 3 0 0  
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3 0 0  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total Cash Equivalents and Short-term Investments 23,142,000   41,739,000
Derivative Asset, Fair Value, Gross Asset 0 [1]   0 [1]
Assets, Fair Value Disclosure, Recurring 23,142,000   41,739,000
Derivative Liability, Fair Value, Gross Liability 0 [2]   0 [3]
Liabilities, Fair Value Disclosure, Recurring 0   0
Stable Net Asset Value Per Share of Money Market Fund and Time Deposits $ 1   $ 1
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total Cash Equivalents and Short-term Investments 223,705,000   326,735,000
Derivative Asset, Fair Value, Gross Asset 555,000 [1]   310,000 [1]
Assets, Fair Value Disclosure, Recurring 224,260,000   327,045,000
Derivative Liability, Fair Value, Gross Liability 204,000 [2]   915,000 [3]
Liabilities, Fair Value Disclosure, Recurring 204,000   915,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total Cash Equivalents and Short-term Investments 0   0
Derivative Asset, Fair Value, Gross Asset 0 [1]   0 [1]
Assets, Fair Value Disclosure, Recurring 0   0
Derivative Liability, Fair Value, Gross Liability 0 [2]   0 [3]
Liabilities, Fair Value Disclosure, Recurring 0   0
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total Cash Equivalents and Short-term Investments 246,847,000   368,474,000
Derivative Asset, Fair Value, Gross Asset 555,000 [1]   310,000 [1]
Assets, Fair Value Disclosure, Recurring 247,402,000   368,784,000
Derivative Liability, Fair Value, Gross Liability 204,000 [2]   915,000 [3]
Liabilities, Fair Value Disclosure, Recurring 204,000   915,000
Time Deposits [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Other Short-term Investments, Fair Value Disclosure 21,773,000   26,395,000
Time Deposits [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Other Short-term Investments, Fair Value Disclosure 21,773,000 [4]   26,395,000 [4]
Time Deposits [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Other Short-term Investments, Fair Value Disclosure 0 [4]   0 [4]
Time Deposits [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Other Short-term Investments, Fair Value Disclosure 0 [4]   0 [4]
Time Deposits [Member] | Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Other Short-term Investments, Fair Value Disclosure 21,773,000 [4]   26,395,000 [4]
Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable Debt Securities, Fair Value Disclosure 0 [4]   0 [4]
Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable Debt Securities, Fair Value Disclosure 223,705,000 [4]   326,735,000 [4]
Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable Debt Securities, Fair Value Disclosure 0 [4]   0 [4]
Available-for-sale Securities [Member] | Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable Debt Securities, Fair Value Disclosure 223,705,000 [4]   326,735,000 [4]
Money Market Funds [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Cash and Cash Equivalents, Fair Value Disclosure 1,369,000   15,344,000
Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Cash and Cash Equivalents, Fair Value Disclosure 1,369,000 [5]   15,344,000 [5]
Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Cash and Cash Equivalents, Fair Value Disclosure 0 [5]   0 [5]
Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Cash and Cash Equivalents, Fair Value Disclosure 0 [5]   0 [5]
Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Cash and Cash Equivalents, Fair Value Disclosure $ 1,369,000 [5]   $ 15,344,000 [5]
[1] Included in prepaid expenses and other current assets on the condensed consolidated balance sheets.
[2] Included in accrued liabilities on the consolidated balance sheets.
[3] Included in accrued liabilities and other liabilities on the consolidated balance sheets.
[4] Included in either cash and cash equivalents or short-term investments on the condensed consolidated balance sheets.
[5] Included in cash and cash equivalents on the condensed consolidated balance sheets.