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Unsecured Convertible Senior Notes - Interest expense recognized (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
2023 Convertible Senior Notes      
Interest Expense, Debt [Abstract]      
Contractual interest expense $ 10,410 $ 13,089 $ 1,677
Amortization of debt issuance costs 669 8,496 996
Amortization of debt discount 7,728 736 86
Total 18,807 $ 22,321 $ 2,759
2026 Convertible Senior Notes      
Interest Expense, Debt [Abstract]      
Contractual interest expense 4,397    
Amortization of debt issuance costs 230    
Amortization of debt discount 3,022    
Total $ 7,649