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Share-Based Compensation Plans (Disclosure of Share-based Compensation Arrangements by Share-based Payment Award) (Details) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Time Based Awards [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Common shares, non-vested as of December 31, 2015 612,037    
Common shares, granted 177,664    
Common shares, vested (310,207)    
Common shares, forfeited (36,342)    
Common shares, non-vested as of December 31, 2016 443,152 612,037  
Weighted average grant date fair value, non-vested as of December 31, 2015 $ 12.32    
Weighted average grant date fair value, granted 18.73 $ 13.52 $ 11.41
Weighted average grant date fair value, vested 12.05    
Weighted average grant date fair value, forfeited 13.23    
Weighted average grant date fair value, non-vested as of December 31, 2016 $ 15.01 $ 12.32  
Performance Based Awards [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Common shares, non-vested as of December 31, 2015 825,140    
Common shares, granted 217,495    
Common shares, vested (165,783)    
Common shares, forfeited (132,664)    
Common shares, non-vested as of December 31, 2016 744,188 825,140  
Weighted average grant date fair value, non-vested as of December 31, 2015 $ 12.14    
Weighted average grant date fair value, granted 21.54    
Weighted average grant date fair value, vested 11.72    
Weighted average grant date fair value, forfeited 12.52    
Weighted average grant date fair value, non-vested as of December 31, 2016 $ 14.92 $ 12.14