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	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 107,899	$ 104,509
Less than 12 Months, Unrealized Losses	156	377
12 Months or Greater, Fair Value	65,120	33,185
12 Months or Greater, Unrealized Losses	86	28
Total, Fair Value	173,019	137,694
Total, Unrealized Losses	242	405
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	14,919	14,237
Less than 12 Months, Unrealized Losses	5	8
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total, Fair Value	14,919	14,237
Total, Unrealized Losses	5	8
Corporate Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	92,500	78,821
Less than 12 Months, Unrealized Losses	151	323
12 Months or Greater, Fair Value	50,152	29,689
12 Months or Greater, Unrealized Losses	56	24
Total, Fair Value	142,652	108,510
Total, Unrealized Losses	207	347
US Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value		11,451
Less than 12 Months, Unrealized Losses		46
12 Months or Greater, Fair Value	14,968	3,496
12 Months or Greater, Unrealized Losses	30	4
Total, Fair Value	14,968	14,947
Total, Unrealized Losses	30	$ 50
Bank Deposits [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	480
12 Months or Greater, Fair Value	0
12 Months or Greater, Unrealized Losses	0
Total, Fair Value	480
Total, Unrealized Losses	$ 0