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Stock-Based Compensation Plans - Narrative (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Performance-based shares earned         142,256
Stock-based compensation cost $ 1.9 $ 2.7 $ 5.5 $ 6.7  
Total unrecognized compensation cost related to non-vested share-based compensation arrangements $ 8.2   $ 8.2    
Compensation cost not yet recognized, period for recognition     1 year 6 months 2 days    
Restricted Stock Units          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Granted, number of shares     124,077 124,170  
Grants in period, weighted average grant date fair value (in dollars per share)     $ 38.21 $ 32.47  
Performance Shares          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Granted, number of shares     156,576   180,526
Grants in period, weighted average grant date fair value (in dollars per share)     $ 38.47   $ 32.60