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Long-Term Debt - Convertible Senior Notes Due 2022 (Details)
$ / shares in Units, $ in Thousands
Nov. 01, 2016
USD ($)
$ / shares
Sep. 30, 2018
USD ($)
Dec. 31, 2017
USD ($)
Debt Instrument [Line Items]      
Principal amount   $ 489,025  
Unamortized debt discount   30,300  
Convertible Senior Notes Maturing May 2022      
Debt Instrument [Line Items]      
Principal amount $ 125,000 125,000 $ 125,000
Interest rate (as a percent) 4.25%    
Frequency of periodic payment semi-annually    
Maturity date May 01, 2022    
Initial conversion ratio 0.0719748    
Initial conversion price per share (in dollars per share) | $ / shares $ 13.89    
Redemption price as a percentage of principal amount 100.00%    
Minimum percentage in aggregate principal amount 25.00%    
Unamortized debt discount $ 16,900 $ 11,772 $ 13,900
Carrying amount of equity component $ 11,000    
Effective interest rate (as a percent) 7.30%