Investments and Fair Value Measurements - Amortized Cost and Estimated Fair Value of Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Amortized Cost and Fair Value Debt Securities [Abstract]
Mature in less than one year, Amortized cost	$ 56,666
Mature in one to three years, Amortized cost	85,634
Amortized cost	142,300	$ 149,815
Mature in less than one year, Fair value	56,348
Mature in one to three years, Fair value	84,311
Total short-term investments, Fair value	$ 140,659	$ 148,697