Cash, Cash Equivalents, Restricted Cash and Short-Term Investments - Summary of Cash, Cash Equivalents and Short-Term Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Cash and cash equivalents, Amortized Cost	$ 209,194	$ 192,541
Cash and cash equivalents, Gross Unrealized Gains	0	0
Cash and cash equivalents, Gross Unrealized Losses	0	3
Cash and cash equivalents, Fair Value	209,194	192,538
Short-term investments, Amortized Cost	186,114	167,596
Short-term investments, Gross Unrealized Gains	8	0
Short-term investments, Gross Unrealized Losses	844	603
Short-term investments, Estimated Fair Value	185,278	166,993
Cash [Member]
Schedule of Investments [Line Items]
Cash and cash equivalents, Amortized Cost	64,042	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	64,042	92,030
Money Market Funds [Member]
Schedule of Investments [Line Items]
Cash and cash equivalents, Amortized Cost	145,152	84,144
Cash and cash equivalents, Gross Unrealized Gains	0	0
Cash and cash equivalents, Gross Unrealized Losses	0	0
Cash and cash equivalents, Fair Value	145,152	84,144
U.S. Government and U.S. Government Agencies [Member]
Schedule of Investments [Line Items]
Cash and cash equivalents, Amortized Cost	0	16,367
Cash and cash equivalents, Gross Unrealized Gains	0	0
Cash and cash equivalents, Gross Unrealized Losses	0	3
Cash and cash equivalents, Fair Value	0	16,364
Short-term investments, Amortized Cost	60,012	43,097
Short-term investments, Gross Unrealized Gains	7	0
Short-term investments, Gross Unrealized Losses	172	114
Short-term investments, Estimated Fair Value	59,847	42,983
Commercial Paper [Member]
Schedule of Investments [Line Items]
Short-term investments, Amortized Cost	1,492	699
Short-term investments, Gross Unrealized Gains	1	0
Short-term investments, Gross Unrealized Losses	2	0
Short-term investments, Estimated Fair Value	1,491	699
Corporate Debt Obligations [Member]
Schedule of Investments [Line Items]
Short-term investments, Amortized Cost	124,370	123,320
Short-term investments, Gross Unrealized Gains	0	0
Short-term investments, Gross Unrealized Losses	669	488
Short-term investments, Estimated Fair Value	123,701	122,832
Certificate of Deposits [Member]
Schedule of Investments [Line Items]
Short-term investments, Amortized Cost	240	480
Short-term investments, Gross Unrealized Gains	0	0
Short-term investments, Gross Unrealized Losses	1	1
Short-term investments, Estimated Fair Value	$ 239	$ 479