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Earnings Per Share (Details) - USD ($)
shares in Millions, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Jun. 30, 2011
Restricted stock        
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]        
Antidilutive securities 5.2 4.3 3.7  
Warrants        
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]        
Antidilutive securities 10.8 10.8 10.8  
Convertible debt        
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]        
Principal amount $ 316.3      
Convertible debt | Convertible Senior Notes Due June 2018        
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]        
Principal amount       $ 316.3
Interest rate       2.75%
Maximum number of equity instruments 3.0      
Convertible debt | Convertible Senior Notes Due June 2018 | Convertible debt securities        
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]        
Antidilutive securities 13.8 13.8 13.8