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Stock-Based Compensation Plans - Narrative (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2016
Jun. 30, 2015
Jun. 30, 2016
Jun. 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 $ 1.7 $ 2.1 $ 3.6 $ 4.0  
Total unrecognized compensation cost related to non-vested share-based compensation arrangements $ 9.9   $ 9.9    
Compensation cost not yet recognized, period for recognition     1 year 8 months 2 days    
Restricted Stock Units          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Granted, number of shares     117,620 124,170  
Grants in period, weighted average grant date fair value (in dollars per share)     $ 38.36 $ 32.51  
Performance Shares          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Granted, number of shares     156,572   180,526
Grants in period, weighted average grant date fair value (in dollars per share)     $ 38.47   $ 32.60