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Earnings Per Share - Additional Information (Details)
12 Months Ended
Dec. 31, 2013
8.5% Convertible perpetual preferred stock
 
Earnings Per Share Disclosure [Line Items]  
Preferred stock, dividend rate, percentage 8.50%
6.0% Convertible perpetual preferred stock
 
Earnings Per Share Disclosure [Line Items]  
Preferred stock, dividend rate, percentage 6.00%
7.0% Convertible perpetual preferred stock
 
Earnings Per Share Disclosure [Line Items]  
Preferred stock, dividend rate, percentage 7.00%