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Investments - Estimated Fair Value and Gross Unrealized Loss Position for AFS Debt Securities (Detail) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]    
Less Than 12 Months, Fair Value $ 142,545 $ 331,622
Less Than 12 Months, Unrealized Losses (71) (792)
12 Months or Greater, Fair Value 42,451 245,745
12 Months or Greater, Unrealized Losses (11) (1,376)
Total Fair Value 184,996 577,367
Total Unrealized Losses (82) (2,168)
Commercial paper    
Schedule Of Available For Sale Securities [Line Items]    
Less Than 12 Months, Fair Value 28,839 51,927
Less Than 12 Months, Unrealized Losses (4) (45)
Total Fair Value 28,839 51,927
Total Unrealized Losses (4) (45)
Corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Less Than 12 Months, Fair Value 84,753 274,696
Less Than 12 Months, Unrealized Losses (63) (746)
12 Months or Greater, Fair Value 42,451 234,798
12 Months or Greater, Unrealized Losses (11) (1,321)
Total Fair Value 127,204 509,494
Total Unrealized Losses (74) (2,067)
Securities of government-sponsored entities    
Schedule Of Available For Sale Securities [Line Items]    
Less Than 12 Months, Fair Value 28,953 4,999
Less Than 12 Months, Unrealized Losses (4) (1)
12 Months or Greater, Fair Value   10,947
12 Months or Greater, Unrealized Losses   (55)
Total Fair Value 28,953 15,946
Total Unrealized Losses $ (4) $ (56)