Summary of Continuous Unrealized Loss on Marketable Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule Of Held To Maturity Securities [Line Items]
Estimated Fair Value, Less Than 12 Months	$ 141,165	$ 145,538
Unrealized Loss, Less Than 12 Months	(241)	(146)
Estimated Fair Value, 12 Months or Greater	0	0
Unrealized Loss, 12 Months or Greater	0	0
Estimated Fair Value, Total	141,165	145,538
Unrealized Loss, Total	(241)	(146)
Commercial Paper
Schedule Of Held To Maturity Securities [Line Items]
Estimated Fair Value, Less Than 12 Months	136,283	113,586
Unrealized Loss, Less Than 12 Months	(240)	(107)
Estimated Fair Value, 12 Months or Greater	0	0
Unrealized Loss, 12 Months or Greater	0	0
Estimated Fair Value, Total	136,283	113,586
Unrealized Loss, Total	(240)	(107)
Corporate Bonds
Schedule Of Held To Maturity Securities [Line Items]
Estimated Fair Value, Less Than 12 Months	4,882	31,952
Unrealized Loss, Less Than 12 Months	(1)	(39)
Estimated Fair Value, 12 Months or Greater	0	0
Unrealized Loss, 12 Months or Greater	0	0
Estimated Fair Value, Total	4,882	31,952
Unrealized Loss, Total	$ (1)	$ (39)