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Financing Arrangements - Expenses in Relation to Convertible Notes (Details) - USD ($)
$ in Thousands
12 Months Ended
Apr. 06, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Jun. 01, 2011
Debt Instrument [Line Items]          
Deferred financing cost amortization   $ 5,216 $ 10,791 $ 4,283  
Convertible Debt          
Debt Instrument [Line Items]          
Interest expense at 3.50% coupon rate   0 687 2,205  
Debt discount amortization   0 750 2,421  
Deferred financing cost amortization   0 136 438  
Loss on conversion   0 0 1,235  
Interest expense   $ 0 $ 1,573 $ 6,299  
Coupon Rate         3.50%
Debt Instrument, Redemption, Period One | Convertible Debt          
Debt Instrument [Line Items]          
Conversion penalty percentage 0.50%        
Debt Instrument, Redemption, Period Two | Convertible Debt          
Debt Instrument [Line Items]          
Conversion penalty percentage 0.25%