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Financial Instruments and Fair Value Measurements - Available-for-Sale Securities (Details) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2020
Dec. 31, 2019
Available-for-Sale Securities    
Amortized Cost $ 382,465 $ 306,706
Gross Unrealized Gains 6 35
Gross Unrealized Losses 0 0
Estimated Fair Value $ 382,471 306,741
Debt securities, contractual maturities 1 year  
Debt securities, average duration 1 month  
Credit loss, available-for-sale debt securities $ 0  
U.S. government securities    
Available-for-Sale Securities    
Amortized Cost 3,993 53,799
Gross Unrealized Gains 6 35
Gross Unrealized Losses 0 0
Estimated Fair Value 3,999 53,834
U.S. commercial paper    
Available-for-Sale Securities    
Amortized Cost   18,915
Gross Unrealized Gains   0
Gross Unrealized Losses   0
Estimated Fair Value   18,915
Money market funds    
Available-for-Sale Securities    
Amortized Cost 378,472 233,992
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Estimated Fair Value $ 378,472 $ 233,992