Summary of Held-to-Maturity Marketable Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule Of Held To Maturity Securities [Line Items]
Amortized Cost	$ 154,614	$ 164,192
Unrealized Gains	4	17
Unrealized Losses	(241)	(146)
Estimated Fair Value	154,377	164,063
Commercial Paper | Cash and Cash Equivalents
Schedule Of Held To Maturity Securities [Line Items]
Amortized Cost	35,871	22,744
Unrealized Losses	(21)	(5)
Estimated Fair Value	35,850	22,739
Commercial Paper | Short Term Investments
Schedule Of Held To Maturity Securities [Line Items]
Amortized Cost	100,652	90,949
Unrealized Losses	(219)	(102)
Estimated Fair Value	100,433	90,847
Corporate Bonds | Short Term Investments
Schedule Of Held To Maturity Securities [Line Items]
Amortized Cost	18,091	41,503
Unrealized Gains	4	9
Unrealized Losses	(1)	(39)
Estimated Fair Value	$ 18,094	41,473
U.S. Government Agency Bonds | Short Term Investments
Schedule Of Held To Maturity Securities [Line Items]
Amortized Cost		8,996
Unrealized Gains		8
Estimated Fair Value		$ 9,004