Cash, Cash Equivalents, Restricted Cash and Short-Term Investments - Summary of Cost and Estimated Fair Value of Short-Term Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Short Term Investments [Abstract]
Amortized Cost of short-term investments, Due within one year			$ 110,202
Amortized Cost of short-term investments, Due within two years			55,332
Short-term investments, Amortized Cost	$ 165,534	$ 167,596	165,534
Estimated fair value of short-term investments, Due within one year			109,734
Estimated fair value of short-term investments, Due within two years			54,801
Short-term investments, Estimated Fair Value	$ 164,535	$ 166,993	$ 164,535