Debt - Narrative - Convertible Senior Notes (Details) (Convertible Senior Notes [Member], USD $)
0 Months Ended
May 20, 2014
Dec. 31, 2014
May 20, 2014
Debt Instrument [Line Items]      
Initial conversion rate applicable to each series 0.0763636    
Conversion price (in dollars per share) $ 13.10   $ 13.10
Premium above public offering price 22.50%   22.50%
Value of the embedded conversion premium, included in net debt (discount) $ 56,700,000 $ 51,500,000 $ 56,700,000
Deferred finance costs   19,600,000  
Convertible Senior Notes Due 2019 [Member]
     
Debt Instrument [Line Items]      
Aggregate principal amount of debt 402,500,000.0   402,500,000.0
Weighted average stated interest rate 2.875%   2.875%
Convertible Senior Notes Due 2021 [Member]
     
Debt Instrument [Line Items]      
Aggregate principal amount of debt $ 345,000,000.0   $ 345,000,000.0
Weighted average stated interest rate 3.75%   3.75%