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Available-for-Sale Securities and Fair Value Measurements - Available-for-Sale Securities (Details) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2019
Dec. 31, 2018
Available-for-Sale Securities    
Amortized Cost $ 228,259 $ 108,977
Gross Unrealized Gains 33 0
Gross Unrealized Losses 0 (3)
Estimated Fair Value $ 228,292 108,974
Maturity period for marketable securities    
Maximum contractual maturity 1 year  
Weighted average maturity 2 months  
U.S. government securities    
Available-for-Sale Securities    
Amortized Cost $ 49,012 29,736
Gross Unrealized Gains 25 0
Gross Unrealized Losses 0 (3)
Estimated Fair Value 49,037 29,733
U.S. government agencies    
Available-for-Sale Securities    
Amortized Cost 19,919 4,971
Gross Unrealized Gains 8 0
Gross Unrealized Losses 0 0
Estimated Fair Value 19,927 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 38,747 22,037
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Estimated Fair Value 38,747 22,037
Money market funds    
Available-for-Sale Securities    
Amortized Cost 120,581 49,358
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Estimated Fair Value $ 120,581 $ 49,358