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Share-Based Payments (Summary of Share Unit Activity) (Details) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Performance Share Units, Equity Classified Awards [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Outstanding beginning balance, Shares 243,968    
Granted (shares) 136,073 7,714  
Transferred to (from) equity awards on modification, Number of units 81,533 257,934  
Vested, Shares (27,326)    
Canceled, Shares   (21,680)  
Outstanding ending balance, Shares 434,248 243,968  
Outstanding beginning balance, Weighted average grant date fair value (per share) $ 27.48    
Granted, Weighted average grant date fair value (per share) 30.28 $ 31.40  
Transferred to (from) equity awards on modification, WA grant date fair value (per share) 24.47 27.34  
Vested and settled, Weighted average grant date fair value (per share) 26.82    
Forfeited, Weighted average grant date fair value (per share)   27.43  
Outstanding ending balance, Weighted average grant date fair value (per share) $ 27.83 $ 27.48  
Restricted Stock Units, Equity Classified [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Granted (shares) 125,152    
Outstanding ending balance, Shares 125,152    
Granted, Weighted average grant date fair value (per share) $ 26.93    
Outstanding ending balance, Weighted average grant date fair value (per share) $ 26.93    
Performance Share Units, Liability Classified Awards [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Outstanding beginning balance, Shares 311,329 421,585 238,573
Granted (shares) 98,775 257,117 218,699
Transferred to (from) equity awards on modification, Number of units (81,533) (257,934)  
Vested, Shares (49,873) (68,683) (6,870)
Canceled, Shares (19,457) (40,756) (28,817)
Outstanding ending balance, Shares 259,241 311,329 421,585
Outstanding beginning balance, Weighted average grant date fair value (per share) $ 23.96 $ 24.03 $ 23.38
Granted, Weighted average grant date fair value (per share) 31.21 23.32 24.57
Transferred to (from) equity awards on modification, WA grant date fair value (per share) 24.66 23.86  
Vested and settled, Weighted average grant date fair value (per share) 23.64 23.08 22.22
Forfeited, Weighted average grant date fair value (per share) 26.39 22.75 23.23
Outstanding ending balance, Weighted average grant date fair value (per share) $ 26.38 $ 23.96 $ 24.03
Restricted Stock Units, Liability Classified Awards [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Outstanding beginning balance, Shares 160,009 332,954 403,587
Granted (shares) 878 4,543 20,528
Vested, Shares (156,221) (162,306) (28,887)
Canceled, Shares   (15,182) (62,274)
Outstanding ending balance, Shares 4,666 160,009 332,954
Outstanding beginning balance, Weighted average grant date fair value (per share) $ 23.37 $ 22.70 $ 22.32
Granted, Weighted average grant date fair value (per share) 32.30 29.33 26.38
Vested and settled, Weighted average grant date fair value (per share) 23.33 22.23 22.53
Forfeited, Weighted average grant date fair value (per share)   22.68 21.56
Outstanding ending balance, Weighted average grant date fair value (per share) $ 26.42 $ 23.37 $ 22.70
Deferred Share Units [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Outstanding beginning balance, Shares 73,520 57,996 42,289
Granted (shares) 19,967 17,371 29,072
Vested, Shares   (1,847) (13,365)
Outstanding ending balance, Shares 93,487 73,520 57,996
Outstanding beginning balance, Weighted average grant date fair value (per share) $ 25.41 $ 24.21 $ 22.33
Granted, Weighted average grant date fair value (per share) 29.67 29.41 26.07
Vested and settled, Weighted average grant date fair value (per share)   25.28 22.34
Outstanding ending balance, Weighted average grant date fair value (per share) $ 26.32 $ 25.41 $ 24.21