Investments and Fair Value Measurements - Amortized Cost and Estimated Fair Value of Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Amortized Cost and Fair Value Debt Securities [Abstract]
Mature in less than one year, Amortized cost	$ 183,451
Mature in one to three years, Amortized cost	128,562
Amortized cost	312,013	$ 318,820
Mature in less than one year, Fair value	183,666
Mature in one to three years, Fair value	128,560
Total short-term investments, Fair value	$ 312,226	$ 318,599