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Debt - Convertible Notes (Details) - 3.00% Convertible senior notes due 2022 - Convertible debt - USD ($)
Jun. 30, 2019
Dec. 31, 2018
Aug. 31, 2015
Long-term debt      
Principal $ 150,000,000 $ 150,000,000 $ 150,000,000.0
Less: Debt issuance costs (1,544,000) (1,746,000)  
Less: Debt discount, net (30,999,000) (35,054,000)  
Net carrying amount $ 117,457,000 $ 113,200,000