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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, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Stock-based compensation cost $ 2.1 $ 1.5 $ 4.0 $ 2.8  
Total unrecognized compensation cost related to non-vested share-based compensation arrangements $ 9.5   $ 9.5    
Expected period to recognize total unrecognized compensation cost related to non-vested share-based compensation arrangements     1 year 9 months 18 days    
Restricted Stock Units          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Granted, number of shares     62,085 96,572  
Grants in period, weighted average grant date fair value (in dollars per share)     $ 65.01 $ 37.16  
Performance Shares          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Granted, number of shares     91,010   138,800
Grants in period, weighted average grant date fair value (in dollars per share)     $ 65.14   $ 36.79
Performance-based shares earned         116,800