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	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 134,360	$ 104,509
Less than 12 Months, Unrealized Losses	318	377
12 Months or Greater, Fair Value	33,444	33,185
12 Months or Greater, Unrealized Losses	42	28
Total, Fair Value	167,804	137,694
Total, Unrealized Losses	360	405
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	14,215	14,237
Less than 12 Months, Unrealized Losses	8	8
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total, Fair Value	14,215	14,237
Total, Unrealized Losses	8	8
Corporate Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	108,446	78,821
Less than 12 Months, Unrealized Losses	271	323
12 Months or Greater, Fair Value	25,948	29,689
12 Months or Greater, Unrealized Losses	38	24
Total, Fair Value	134,394	108,510
Total, Unrealized Losses	309	347
US Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	11,459	11,451
Less than 12 Months, Unrealized Losses	38	46
12 Months or Greater, Fair Value	7,496	3,496
12 Months or Greater, Unrealized Losses	4	4
Total, Fair Value	18,955	14,947
Total, Unrealized Losses	42	$ 50
Bank Deposits [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	240
Less than 12 Months, Unrealized Losses	1
12 Months or Greater, Fair Value	0
12 Months or Greater, Unrealized Losses	0
Total, Fair Value	240
Total, Unrealized Losses	$ 1