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Available-for-Sale Securities and Fair Value Measurements - Available-for-Sale Securities (Details) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2019
Dec. 31, 2018
Available-for-Sale Securities    
Amortized Cost $ 247,432 $ 108,977
Gross Unrealized Gains 26 0
Gross Unrealized Losses (1) (3)
Estimated Fair Value $ 247,457 108,974
Maturity period for marketable securities    
Maximum contractual maturity 1 year  
Weighted average maturity 4 months  
U.S. government securities    
Available-for-Sale Securities    
Amortized Cost $ 91,440 29,736
Gross Unrealized Gains 26 0
Gross Unrealized Losses (1) (3)
Estimated Fair Value 91,465 29,733
U.S. government agencies    
Available-for-Sale Securities    
Amortized Cost   4,971
Gross Unrealized Gains   0
Gross Unrealized Losses   0
Estimated Fair Value   4,971
U.S. corporate notes    
Available-for-Sale Securities    
Amortized Cost   2,875
Gross Unrealized Gains   0
Gross Unrealized Losses   0
Estimated Fair Value   2,875
U.S. commercial paper    
Available-for-Sale Securities    
Amortized Cost 54,356 22,037
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Estimated Fair Value 54,356 22,037
Money market funds    
Available-for-Sale Securities    
Amortized Cost 101,636 49,358
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Estimated Fair Value $ 101,636 $ 49,358