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Debt (Details) - 3.00% Convertible senior notes due 2022 - Convertible debt - USD ($)
Jun. 30, 2017
Dec. 31, 2016
Aug. 31, 2015
Long-term debt      
Principal $ 150,000,000 $ 150,000,000 $ 150,000,000.0
Less: Debt issuance costs (2,294,000) (2,457,000)  
Less: Debt discount, net (46,054,000) (49,327,000)  
Net carrying amount $ 101,652,000 $ 98,216,000