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Financial Instruments and Fair Value Measurements - Available-for-Sale Securities (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2020
Dec. 31, 2019
Available-for-Sale Securities    
Amortized Cost $ 348,231 $ 306,706
Gross Unrealized Gains 41 35
Gross Unrealized Losses 0 0
Estimated Fair Value $ 348,272 306,741
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 13,958 53,799
Gross Unrealized Gains 41 35
Gross Unrealized Losses 0 0
Estimated Fair Value 13,999 53,834
U.S. commercial paper    
Available-for-Sale Securities    
Amortized Cost 48,964 18,915
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Estimated Fair Value 48,964 18,915
Money market funds    
Available-for-Sale Securities    
Amortized Cost 285,309 233,992
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Estimated Fair Value $ 285,309 $ 233,992