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	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 82,720	$ 107,507
Less than 12 Months, Unrealized Losses	665	453
12 Months or Greater, Fair Value	81,815	57,057
12 Months or Greater, Unrealized Losses	334	150
Total, Fair Value	164,535	164,564
Total, Unrealized Losses	999	603
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value		699
Less than 12 Months, Unrealized Losses		0
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total, Fair Value	0	699
Total, Unrealized Losses	0	0
Corporate Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	47,967	77,715
Less than 12 Months, Unrealized Losses	492	377
12 Months or Greater, Fair Value	69,863	45,117
12 Months or Greater, Unrealized Losses	307	111
Total, Fair Value	117,830	122,832
Total, Unrealized Losses	799	488
US Government and US Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	34,753	29,093
Less than 12 Months, Unrealized Losses	173	76
12 Months or Greater, Fair Value	11,473	11,461
12 Months or Greater, Unrealized Losses	26	38
Total, Fair Value	46,226	40,554
Total, Unrealized Losses	199	114
Bank Deposits [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value		0
Less than 12 Months, Unrealized Losses	0	0
12 Months or Greater, Fair Value	479	479
12 Months or Greater, Unrealized Losses	1	1
Total, Fair Value	479	479
Total, Unrealized Losses	$ 1	$ 1