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Share-Based Compensation Plans - Deferred Stock (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation $ 27.1 $ 31.1 $ 40.6
Deferred stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation 1.1 0.8 0.7
Restricted stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation $ 13.8 $ 9.9 $ 12.9
In lieu of annual cash retainer | Deferred stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock units granted at period end (in shares) 39,805 36,091 33,985
Additional compensation for board service | Deferred stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock units granted at period end (in shares) 8,388 7,920 5,199
Additional compensation for board service | Restricted stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock units granted at period end (in shares) 9,966 13,860 10,248
Stock units canceled   1,752