Summary of Gross Unrealized Losses and Fair Values on Investments (Detail) - USD ($) $ in Thousands |
Dec. 31, 2016 |
Dec. 31, 2015 |
|---|---|---|
| Investments, Debt and Equity Securities [Abstract] | ||
| Less than 12 months, Fair value | $ 48,663 | $ 32,695 |
| Available-for-sale securities- matures within one year, Gross unrealized losses | (557) | (389) |
| 12 months or greater, Fair value | 4,875 | 14,488 |
| Available-for-sale securities- matures after one year through three years, Gross unrealized losses | $ (29) | $ (105) |