Gross Unrealized Losses and Fair Value Available-For-Sale Investments in Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Less Than 12 Months, Estimated Fair Value	$ 94,780	$ 331,622
Less Than 12 Months, Unrealized Losses	(91)	(792)
12 Months or Greater, Estimated Fair Value	185,185	245,745
12 Months or Greater, Unrealized Losses	(415)	(1,376)
Total Estimated Fair Value	279,965	577,367
Total Unrealized Losses	(506)	(2,168)
Commercial paper
Schedule Of Available For Sale Securities [Line Items]
Less Than 12 Months, Estimated Fair Value	31,457	51,927
Less Than 12 Months, Unrealized Losses	(41)	(45)
12 Months or Greater, Estimated Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total Estimated Fair Value	31,457	51,927
Total Unrealized Losses	(41)	(45)
Corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Less Than 12 Months, Estimated Fair Value	63,323	274,696
Less Than 12 Months, Unrealized Losses	(50)	(746)
12 Months or Greater, Estimated Fair Value	174,206	234,798
12 Months or Greater, Unrealized Losses	(394)	(1,321)
Total Estimated Fair Value	237,529	509,494
Total Unrealized Losses	(444)	(2,067)
Securities of government-sponsored entities
Schedule Of Available For Sale Securities [Line Items]
Less Than 12 Months, Estimated Fair Value	0	4,999
Less Than 12 Months, Unrealized Losses	0	(1)
12 Months or Greater, Estimated Fair Value	10,979	10,947
12 Months or Greater, Unrealized Losses	(21)	(55)
Total Estimated Fair Value	10,979	15,946
Total Unrealized Losses	$ (21)	$ (56)