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, 2016	Dec. 31, 2015
Schedule Of Available For Sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 80,705	$ 73,673
Less than 12 Months, Unrealized Losses	160	286
12 Months or Greater, Fair Value	38,676	10,609
12 Months or Greater, Unrealized Losses	39	14
Total, Fair Value	119,381	84,282
Total, Unrealized Losses	199	300
Commercial Paper [Member]
Schedule Of Available For Sale Securities [Line Items]
Less than 12 Months, Fair Value	19,184
Less than 12 Months, Unrealized Losses	16
12 Months or Greater, Fair Value	0
12 Months or Greater, Unrealized Losses	0
Total, Fair Value	19,184
Total, Unrealized Losses	16
Corporate Debt Obligations [Member]
Schedule Of Available For Sale Securities [Line Items]
Less than 12 Months, Fair Value	51,523	73,673
Less than 12 Months, Unrealized Losses	137	286
12 Months or Greater, Fair Value	38,676	10,609
12 Months or Greater, Unrealized Losses	39	14
Total, Fair Value	90,199	84,282
Total, Unrealized Losses	176	$ 300
US Government Agencies [Member]
Schedule Of Available For Sale Securities [Line Items]
Less than 12 Months, Fair Value	9,998
Less than 12 Months, Unrealized Losses	7
12 Months or Greater, Fair Value	0
12 Months or Greater, Unrealized Losses	0
Total, Fair Value	9,998
Total, Unrealized Losses	$ 7