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Stockholders' Equity - Basic and Diluted Shares Reconciliation (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Numerator (basic and diluted):                      
Net income $ 21.7 $ 25.8 $ 23.4 $ 18.8 $ 18.5 $ 23.0 $ 25.0 $ 23.2 $ 89.7 $ 89.7 $ 76.1
Less: Dividends paid to participating securities                 (0.2) (0.3) (0.2)
Less: Undistributed earnings available to participating securities                 (0.3) (0.5) (0.4)
Undistributed and distributed earnings available to common stockholders                 $ 89.2 $ 88.9 $ 75.5
Denominator:                      
Average number of common shares outstanding (shares)                 30,251,400 30,238,000 31,056,700
Effect of dilutive stock-based compensation (shares)                 122,900 118,500 181,600
Average number of common and potential common shares outstanding (shares)                 30,374,300 30,356,500 31,238,300