Cash, Cash Equivalents and Short-Term Investments - Schedule of Short-Term Investments at Fair Value and Length of Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 107,507	$ 104,509
Less than 12 Months, Unrealized Losses	453	377
12 Months or Greater, Fair Value	57,057	33,185
12 Months or Greater, Unrealized Losses	150	28
Total, Fair Value	164,564	137,694
Total, Unrealized Losses	603	405
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	699	14,237
Less than 12 Months, Unrealized Losses	0	8
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total, Fair Value	699	14,237
Total, Unrealized Losses	0	8
Corporate Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	77,715	78,821
Less than 12 Months, Unrealized Losses	377	323
12 Months or Greater, Fair Value	45,117	29,689
12 Months or Greater, Unrealized Losses	111	24
Total, Fair Value	122,832	108,510
Total, Unrealized Losses	488	347
US Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	29,093	11,451
Less than 12 Months, Unrealized Losses	76	46
12 Months or Greater, Fair Value	11,461	3,496
12 Months or Greater, Unrealized Losses	38	4
Total, Fair Value	40,554	14,947
Total, Unrealized Losses	114	$ 50
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
12 Months or Greater, Fair Value	479
12 Months or Greater, Unrealized Losses	1
Total, Fair Value	479
Total, Unrealized Losses	$ 1