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Long-Term Debt - Convertible Senior Notes Due 2022 (Details)
$ / shares in Units, $ in Thousands
3 Months Ended
Nov. 01, 2016
USD ($)
$ / shares
Mar. 31, 2019
USD ($)
Mar. 31, 2018
USD ($)
Dec. 31, 2018
USD ($)
Debt Instrument [Line Items]        
Principal amount   $ 465,909    
Unamortized debt discount   27,289    
Interest expense   7,896 $ 8,299  
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 10,305   $ 11,000
Carrying amount of equity component $ 11,000      
Effective interest rate (as a percent) 7.30%      
Interest expense   $ 2,100 $ 2,000