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Share-Based Compensation Plans - Performance Share Units (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Payout Levels (in units):        
Share-based compensation $ 28.7 $ 27.1 $ 31.1  
Performance share units        
Payout Levels (in units):        
Threshold Award (in shares) 57,563 65,141 82,298 94,608
Target Award (in shares) 115,125 130,282 164,595 189,215
Outstanding Award (in shares) 230,250 260,564 329,190 378,430
Share-based compensation $ 10.8 $ 9.1 $ 17.1  
Performance share units | February 11, 2015        
Payout Levels (in units):        
Payout Achieved Over Performance Period (in shares) 219,519      
Performance share units | February 11, 2014 and May 1, 2014        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares Issued (in shares) 335,179      
Minimum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Performance period range 1 year      
Maximum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Performance period range 3 years