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Share-Based Employee Compensation - Roll-Forward of Activity for Performance Share Units (Details) - Performance shares - $ / shares
shares in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Target Award Achieved, Shares 1,306    
Number of Share units      
Nonvested number of Units, beginning balance 1,579    
Granted, number of units 462 420 510
Vested, number of units (566)    
Forfeited, number of units (42)    
Nonvested, Number of Shares, Ending Balance 1,433    
Weighted Average Grant Date Fair Value      
Nonvested weighted Average Grant Date Fair Value, beginning balance (in dollars per unit) $ 8.24    
Granted, weighted average grant date fair value (in dollars per unit) 10.68 $ 8.61 $ 7.96
Vested, weighted average grant date fair value (in dollars per units) 8.30    
Forfeited, weighted average grant date fair value (in dollars per unit) 9.45    
Nonvested weighted Average Grant Date Fair Value, ending balance (in dollars per unit) $ 9.05 $ 8.24  
Award requisite service period 3 years 1 year 1 year
Minimum      
Weighted Average Grant Date Fair Value      
Award requisite service period 1 year    
Target award shares percent 50.00%    
Maximum      
Weighted Average Grant Date Fair Value      
Award requisite service period 3 years    
Target award shares percent 80.00%