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Fair value of financial instruments and investments - Available-for-sale securities (Details) - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]    
Unrealized losses $ (28) $ (8)
Fair Value 95,990 16,223
Unrealized losses 0 (3)
Fair Value 0 10,087
Unrealized losses (28) (11)
Fair Value 95,990 26,310
Commercial paper    
Debt Securities, Available-for-sale [Line Items]    
Unrealized losses   (1)
Fair Value   1,993
Unrealized losses   0
Fair Value   0
Unrealized losses   (1)
Fair Value   1,993
Corporate debt securities    
Debt Securities, Available-for-sale [Line Items]    
Unrealized losses (20) (7)
Fair Value 71,779 14,230
Unrealized losses 0 (3)
Fair Value 0 10,087
Unrealized losses (20) (10)
Fair Value 71,779 $ 24,317
Asset-backed Securities    
Debt Securities, Available-for-sale [Line Items]    
Unrealized losses (8)  
Fair Value 24,211  
Unrealized losses 0  
Fair Value 0  
Unrealized losses (8)  
Fair Value $ 24,211