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Debt - Convertible Notes (Details) - Convertible debt - USD ($)
Dec. 31, 2020
Dec. 31, 2019
Sep. 30, 2019
Aug. 31, 2015
3.00% Convertible senior notes due 2022        
Long-term debt        
Principal $ 150,000,000 $ 150,000,000   $ 150,000,000.0
Less: Debt issuance costs (865,000) (1,329,000)    
Less: Debt discount, net (17,372,000) (26,686,000)    
Net carrying amount 131,763,000 121,985,000    
1.50% Convertible senior notes due 2026        
Long-term debt        
Principal 287,500,000 287,500,000 $ 287,500,000  
Less: Debt issuance costs (4,058,000) (4,567,000)    
Less: Debt discount, net (106,065,000) (119,350,000)    
Net carrying amount $ 177,377,000 $ 163,583,000