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Stockholders' Equity - Basic and Diluted Shares Reconciliation (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Numerator (basic and diluted):                      
Net income $ 20.2 $ 27.7 $ 20.5 $ 17.4 $ 7.2 $ 41.0 $ 25.8 $ 20.5 $ 85.8 $ 94.5 $ 34.5
Less: Dividends paid to participating securities                 (0.4) (0.3) (0.4)
Less: Undistributed earnings available to participating securities                 (0.2) (0.3) 0.0
Undistributed and distributed earnings available to common stockholders                 $ 85.2 $ 93.9 $ 34.1
Denominator:                      
Average number of common shares outstanding (shares)                 30,652,200 30,551,300 30,407,100
Effect of dilutive stock-based compensation (shares)                 186,100 141,600 142,200
Average number of common and potential common shares outstanding (shares)                 30,838,300 30,692,900 30,549,300