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Investments - Summary of Gross Unrealized Losses and Fair Value of Available-For-Sale Investment Securities in Unrealized Loss Position (Detail) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value $ 4,000 $ 242,961
Less Than 12 Months, Unrealized Losses (8) (208)
12 Months or Greater, Estimated Fair Value 26,079 44,637
12 Months or Greater, Unrealized Losses (14) (214)
Total, Estimated Fair Value 30,079 287,598
Total, Unrealized Losses (22) (422)
Corporate debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value 4,000 91,265
Less Than 12 Months, Unrealized Losses (8) (130)
12 Months or Greater, Estimated Fair Value 26,079 44,637
12 Months or Greater, Unrealized Losses (14) (214)
Total, Estimated Fair Value 30,079 135,902
Total, Unrealized Losses $ (22) (344)
Commercial paper    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value   151,696
Less Than 12 Months, Unrealized Losses   (78)
Total, Estimated Fair Value   151,696
Total, Unrealized Losses   $ (78)