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Long-Term Debt - Convertible Senior Notes Due 2022 (Details)
$ / shares in Units, $ in Thousands
3 Months Ended 6 Months Ended
Nov. 01, 2016
USD ($)
$ / shares
Jun. 30, 2019
USD ($)
Jun. 30, 2018
USD ($)
Jun. 30, 2019
USD ($)
Jun. 30, 2018
USD ($)
Dec. 31, 2018
USD ($)
Debt Instrument [Line Items]            
Principal amount   $ 459,224   $ 459,224    
Unamortized debt discount   25,732   25,732    
Interest expense   8,045 $ 8,041 15,941 $ 16,340  
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 9,549   9,549   $ 11,000
Carrying amount of equity component $ 11,000          
Effective interest rate (as a percent) 7.30%          
Interest expense   $ 2,100 $ 2,000 $ 4,200 $ 4,000