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, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 104,509	$ 73,673
Less than 12 Months, Unrealized Losses	377	286
12 Months or Greater, Fair Value	33,185	10,609
12 Months or Greater, Unrealized Losses	28	14
Total, Fair Value	137,694	84,282
Total, Unrealized Losses	405	300
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	14,237
Less than 12 Months, Unrealized Losses	8
Total, Fair Value	14,237
Total, Unrealized Losses	8
Corporate Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	78,821	73,673
Less than 12 Months, Unrealized Losses	323	286
12 Months or Greater, Fair Value	29,689	10,609
12 Months or Greater, Unrealized Losses	24	14
Total, Fair Value	108,510	84,282
Total, Unrealized Losses	347	$ 300
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	3,496
12 Months or Greater, Unrealized Losses	4
Total, Fair Value	14,947
Total, Unrealized Losses	$ 50