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Debt - Convertible Senior Notes (Details) - USD ($)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Aug. 12, 2014
Debt Instrument [Line Items]        
Interest expense on borrowings $ 200,000 $ 400,000 $ 1,600,000  
2019 Notes        
Debt Instrument [Line Items]        
Convertible senior notes       $ 230,000,000.0
Interest rate, as a percent       0.75%
Principal amount       $ 230,000,000.0
Debt issuance costs       $ 7,100,000
Interest expense on borrowings $ 0 $ 363,000 $ 1,578,000