Summary of Continuous Unrealized Loss on Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule Of Held To Maturity Securities [Line Items]
Estimated Fair Value, Less Than 12 Months	$ 146,831	$ 145,538
Unrealized Loss, Less Than 12 Months	(160)	(146)
Estimated Fair Value, 12 Months or Greater	0	0
Unrealized Loss, 12 Months or Greater	0	0
Estimated Fair Value, Total	146,831	145,538
Unrealized Loss, Total	(160)	(146)
Commercial Paper
Schedule Of Held To Maturity Securities [Line Items]
Estimated Fair Value, Less Than 12 Months	131,231	113,586
Unrealized Loss, Less Than 12 Months	(153)	(107)
Estimated Fair Value, 12 Months or Greater	0	0
Unrealized Loss, 12 Months or Greater	0	0
Estimated Fair Value, Total	131,231	113,586
Unrealized Loss, Total	(153)	(107)
Corporate Bonds
Schedule Of Held To Maturity Securities [Line Items]
Estimated Fair Value, Less Than 12 Months	15,600	31,952
Unrealized Loss, Less Than 12 Months	(7)	(39)
Estimated Fair Value, 12 Months or Greater	0	0
Unrealized Loss, 12 Months or Greater	0	0
Estimated Fair Value, Total	15,600	31,952
Unrealized Loss, Total	$ (7)	$ (39)