Quarterly report pursuant to Section 13 or 15(d)

Debt Instruments - Schedule of Long-Term Debt (Details)

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Debt Instruments - Schedule of Long-Term Debt (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
May 24, 2018
Dec. 31, 2017
Dec. 09, 2015
Debt Instrument [Line Items]        
Long-term debt, net $ 255,272   $ 273,014  
Convertible debt | 1.75% convertible senior notes due 2020        
Debt Instrument [Line Items]        
1.75% convertible senior notes 152,100   345,000  
Unamortized discount and deferred issuance costs (24,453)   (71,986)  
Long-term debt, net $ 127,647 $ 192,900 273,014  
Interest rate 1.75%     1.75%
Convertible debt | 1.75% convertible senior notes due 2023        
Debt Instrument [Line Items]        
1.75% convertible senior notes $ 192,900   0  
Unamortized discount and deferred issuance costs (65,275)   0  
Long-term debt, net $ 127,625   $ 0  
Interest rate 1.75% 1.75%