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Investments and Fair Value Measurements - Convertible senior notes (Details) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Maturity period for marketable securities      
Maximum contractual maturity period 6 months    
Weighted average contractual maturity period 1 month    
Fair value transfers      
Fair value of assets transferred from Level 1 to Level 2 $ 0   $ 0
Fair value of assets transferred from Level 2 to Level 1 0   $ 0
Unrealized losses      
Net unrealized losses $ 0    
Available-for-sale securities sold   $ 19,900