Cash, Cash Equivalents, Restricted Cash and Short-Term Investments - Schedule of Short-Term Investments at Fair Value and Length of Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 100,490	$ 107,507
Less than 12 Months, Unrealized Losses	498	453
12 Months or Greater, Fair Value	78,370	57,057
12 Months or Greater, Unrealized Losses	346	150
Total, Fair Value	178,860	164,564
Total, Unrealized Losses	844	603
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	1,044	699
Less than 12 Months, Unrealized Losses	2	0
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total, Fair Value	1,044	699
Total, Unrealized Losses	2	0
Corporate Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	53,163	77,715
Less than 12 Months, Unrealized Losses	331	377
12 Months or Greater, Fair Value	70,538	45,117
12 Months or Greater, Unrealized Losses	338	111
Total, Fair Value	123,701	122,832
Total, Unrealized Losses	669	488
U.S. Government and U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	46,283	29,093
Less than 12 Months, Unrealized Losses	165	76
12 Months or Greater, Fair Value	7,593	11,461
12 Months or Greater, Unrealized Losses	7	38
Total, Fair Value	53,876	40,554
Total, Unrealized Losses	172	114
Certificate of Deposits [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	0	0
Less than 12 Months, Unrealized Losses	0	0
12 Months or Greater, Fair Value	239	479
12 Months or Greater, Unrealized Losses	1	1
Total, Fair Value	239	479
Total, Unrealized Losses	$ 1	$ 1