Cash, Cash Equivalents, Restricted Cash and Short-Term Investments - Summary of Cash, Cash Equivalents and Short-Term Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Schedule of Investments [Line Items]
Cash and cash equivalents, Amortized Cost	$ 214,277	$ 192,541
Cash and cash equivalents, Gross Unrealized Gains	0	0
Cash and cash equivalents, Gross Unrealized Losses	3	3
Cash and cash equivalents, Fair Value	214,274	192,538
Short-term investments, Amortized Cost	165,534	167,596	$ 165,534
Short-term investments, Gross Unrealized Gains	0	0
Short-term investments, Gross Unrealized Losses	999	603
Short-term investments, Estimated Fair Value	164,535	166,993	$ 164,535
Cash [Member]
Schedule of Investments [Line Items]
Cash and cash equivalents, Amortized Cost	110,746	92,030
Cash and cash equivalents, Gross Unrealized Gains	0	0
Cash and cash equivalents, Gross Unrealized Losses	0	0
Cash and cash equivalents, Fair Value	110,746	92,030
Commercial Paper [Member]
Schedule of Investments [Line Items]
Cash and cash equivalents, Amortized Cost	3,995	0
Cash and cash equivalents, Gross Unrealized Gains	0	0
Cash and cash equivalents, Gross Unrealized Losses	2	0
Cash and cash equivalents, Fair Value	3,993	0
Short-term investments, Amortized Cost		699
Short-term investments, Gross Unrealized Gains	0	0
Short-term investments, Gross Unrealized Losses	0	0
Short-term investments, Estimated Fair Value	0	699
Money Market Funds [Member]
Schedule of Investments [Line Items]
Cash and cash equivalents, Amortized Cost	84,081	84,144
Cash and cash equivalents, Gross Unrealized Gains	0	0
Cash and cash equivalents, Gross Unrealized Losses		0
Cash and cash equivalents, Fair Value	84,081	84,144
US Government and US Government Agencies [Member]
Schedule of Investments [Line Items]
Cash and cash equivalents, Amortized Cost	15,455	16,367
Cash and cash equivalents, Gross Unrealized Gains	0	0
Cash and cash equivalents, Gross Unrealized Losses	1	3
Cash and cash equivalents, Fair Value	15,454	16,364
Short-term investments, Amortized Cost	46,426	43,097
Short-term investments, Gross Unrealized Gains	0	0
Short-term investments, Gross Unrealized Losses	199	114
Short-term investments, Estimated Fair Value	46,227	42,983
Corporate Debt Obligations [Member]
Schedule of Investments [Line Items]
Short-term investments, Amortized Cost	118,628	123,320
Short-term investments, Gross Unrealized Gains	0	0
Short-term investments, Gross Unrealized Losses	799	488
Short-term investments, Estimated Fair Value	117,829	122,832
Bank Deposits [Member]
Schedule of Investments [Line Items]
Short-term investments, Amortized Cost	480	480
Short-term investments, Gross Unrealized Gains	0	0
Short-term investments, Gross Unrealized Losses	1	1
Short-term investments, Estimated Fair Value	$ 479	$ 479