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Long-Term Debt - Convertible Senior Notes Due 2022 (Details)
$ / shares in Units, $ in Thousands
3 Months Ended 9 Months Ended
Nov. 01, 2016
USD ($)
$ / shares
Sep. 30, 2019
USD ($)
Sep. 30, 2018
USD ($)
Sep. 30, 2019
USD ($)
Sep. 30, 2018
USD ($)
Dec. 31, 2018
USD ($)
Debt Instrument [Line Items]            
Principal amount   $ 445,949   $ 445,949    
Unamortized debt discount   24,160   24,160    
Interest expense   7,694 $ 8,171 23,635 $ 24,511  
Convertible Senior Notes Maturing May 2022            
Debt Instrument [Line Items]            
Principal amount $ 125,000 125,000   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 8,784   8,784   $ 11,000
Carrying amount of equity component $ 11,000          
Effective interest rate (as a percent) 7.30%          
Interest expense   $ 2,100 $ 2,000 $ 6,200 $ 6,100