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Gross Unrealized Losses and Fair Value Available-For-Sale Investments in Unrealized Loss Position (Detail) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value $ 331,622 $ 460,252
Less Than 12 Months, Unrealized Losses (792) (1,777)
12 Months or Greater, Estimated Fair Value 245,745 35,566
12 Months or Greater, Unrealized Losses (1,376) (74)
Total Estimated Fair Value 577,367 495,818
Total Unrealized Losses (2,168) (1,851)
Commercial paper    
Schedule Of Available For Sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value 51,927 62,602
Less Than 12 Months, Unrealized Losses (45) (35)
12 Months or Greater, Estimated Fair Value 0 0
12 Months or Greater, Unrealized Losses 0 0
Total Estimated Fair Value 51,927 62,602
Total Unrealized Losses (45) (35)
Corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value 274,696 386,728
Less Than 12 Months, Unrealized Losses (746) (1,660)
12 Months or Greater, Estimated Fair Value 234,798 28,087
12 Months or Greater, Unrealized Losses (1,321) (50)
Total Estimated Fair Value 509,494 414,815
Total Unrealized Losses (2,067) (1,710)
Securities of government-sponsored entities    
Schedule Of Available For Sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value 4,999 10,922
Less Than 12 Months, Unrealized Losses (1) (82)
12 Months or Greater, Estimated Fair Value 10,947 7,479
12 Months or Greater, Unrealized Losses (55) (24)
Total Estimated Fair Value 15,946 18,401
Total Unrealized Losses $ (56) $ (106)