Investments and Fair Value Measurements - Amortized Cost and Estimated Fair Value of Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Amortized Cost and Fair Value Debt Securities [Abstract]
Mature in less than one year, Amortized cost	$ 56,677
Mature in one to three years, Amortized cost	161,626
Amortized cost	218,303	$ 296,185
Mature in less than one year, Fair value	56,646
Mature in one to three years, Fair value	161,277
Total short-term investments, Fair value	$ 217,923	$ 295,428