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Share-Based Compensation - Performance Units/Performance Share Units - Fair Value Assumptions (Details) - $ / shares
6 Months Ended 12 Months Ended
Jun. 30, 2015
Dec. 31, 2014
Performance Units    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected price volatility range, minimum 25.70% 26.60%
Expected price volatility range, maximum 89.70% 86.60%
Weighted-average risk-free interest rate 0.30% 0.50%
Weighted-average fair value per unit (usd per unit) $ 1.41 $ 13.85
Performance Share Units    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected price volatility range, minimum 29.00%  
Expected price volatility range, maximum 77.20%  
Weighted-average risk-free interest rate 0.90%  
Weighted-average fair value per unit (usd per unit) $ 0.52