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Debt - Convertible Notes (Details) - Convertible debt - USD ($)
Sep. 30, 2019
Dec. 31, 2018
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 (1,438,000) (1,746,000)  
Less: Debt discount, net (28,862,000) (35,054,000)  
Net carrying amount 119,700,000 $ 113,200,000  
1.50% Convertible senior notes due 2026      
Long-term debt      
Principal 287,500,000    
Less: Debt issuance costs (4,692,000)    
Less: Debt discount, net (122,618,000)    
Net carrying amount $ 160,190,000