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Stock-Based Compensation Expense (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Maximum number of shares reserved for grants 6,555,000    
Stock-based compensation expense [Abstract]      
Stock-based compensation expense $ 9.4 $ 10.0 $ 9.3
Less: related tax benefit 2.0 2.1 2.0
Net stock-based compensation expense 7.4 7.9 7.3
Stock Option [Member]      
Stock-based compensation expense [Abstract]      
Stock-based compensation expense 0.0 0.1 0.3
Restricted stock units (RSUs)      
Stock-based compensation expense [Abstract]      
Stock-based compensation expense 3.0 2.7 2.5
Performance shares units [Member]      
Stock-based compensation expense [Abstract]      
Stock-based compensation expense $ 6.4 $ 7.2 $ 6.5