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Gross Unrealized Losses and Fair Value Available-For-Sale Debt securities 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 $ 234,124 $ 331,622
Less Than 12 Months, Unrealized Losses (127) (792)
12 Months or Greater, Estimated Fair Value 0 245,745
12 Months or Greater, Unrealized Losses 0 (1,376)
Total Estimated Fair Value 234,124 577,367
Total Unrealized Losses (127) (2,168)
Commercial paper    
Schedule Of Available For Sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value 33,070 51,927
Less Than 12 Months, Unrealized Losses (15) (45)
12 Months or Greater, Estimated Fair Value 0 0
12 Months or Greater, Unrealized Losses 0 0
Total Estimated Fair Value 33,070 51,927
Total Unrealized Losses (15) (45)
Corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value 186,052 274,696
Less Than 12 Months, Unrealized Losses (110) (746)
12 Months or Greater, Estimated Fair Value 0 234,798
12 Months or Greater, Unrealized Losses 0 (1,321)
Total Estimated Fair Value 186,052 509,494
Total Unrealized Losses (110) (2,067)
Securities of government-sponsored entities    
Schedule Of Available For Sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value 15,002 4,999
Less Than 12 Months, Unrealized Losses (2) (1)
12 Months or Greater, Estimated Fair Value 0 10,947
12 Months or Greater, Unrealized Losses 0 (55)
Total Estimated Fair Value 15,002 15,946
Total Unrealized Losses $ (2) $ (56)