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Net Income Per Share (Tables)
 3 Months Ended
Mar. 31, 2021
Earnings Per Share [Abstract]  
Reconciliation of the Common and Potential Common Shares Outstanding Used in Earnings Per Share Calculation A reconciliation of the average number of common and potential common shares outstanding used in the calculations of basic and diluted net income per share follows ($ in millions, shares in thousands):
Three Months Ended
March 31,
2021		March 31,
2020
Numerator (basic and diluted):  
Net income$21.6 $22.5 
Less: Dividends paid to participating securities(0.1)(0.3)
Less: Undistributed earnings available to participating securities(0.1)— 
Undistributed and distributed earnings available to common stockholders$21.4 $22.2 
Denominator:  
Average number of common shares outstanding30,974.2 30,712.3 
Effect of dilutive stock-based compensation366.3 197.7 
Average number of common and potential common shares outstanding31,340.5 30,910.0