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Share-Based Compensation, Performance Based Awards (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Forfeited, number of shares (14,198) (10,777) (5,995)
Performance Based Awards [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Requisite service period 3 years    
OR and ROIC performance period 3 years    
Revenue growth multiplier performance period 3 years    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Nonvested stock, number of shares, beginning of period [1] 148,185    
Nonvested stock, weighted-average grant date fair value, beginning of period $ 100.76    
Granted, number of shares [1] 49,068    
Granted, weighted-average grant date fair value $ 157.75 $ 110.13 $ 105.83
Vested, number of shares [1] (51,299)    
Vested, weighted-average grant date fair value $ 87.10    
Forfeited, number of shares [1] (5,394)    
Forfeited, weighted-average grant date fair value $ 117.94    
Nonvested stock, number of shares, end of period [1] 140,560 148,185  
Nonvested stock, weighted-average grant date fair value, end of period $ 124.98 $ 100.76  
Compensation expense $ 8.6 $ 8.2 $ 5.8
Income tax benefit recognized in the consolidated statements of income 2.1 2.0 1.5
Unrecognized compensation cost relating to nonvested stock $ 5.3    
Unrecognized compensation cost weighted-average period of recognition 11 months    
Shares vested, fair value $ 7.8 $ 5.7 $ 3.5
Performance Based Awards [Member] | Minimum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Revenue growth multiplier range 80.00%    
Performance based award, earned percentage 0.00%    
Performance Based Awards [Member] | Maximum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Revenue growth multiplier range 125.00%    
Performance based award, earned percentage 200.00%    
2020 Awards [Member] | Performance Based Awards [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Maximum number of shares 88,122    
2019 Awards [Member] | Performance Based Awards [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Maximum number of shares 92,582    
2018 Awards [Member] | Performance Based Awards [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Earned shares 71,074    
[1] * For the 2020 Awards and the 2019 Awards, participants in the aggregate can earn up to a maximum of 88,122 and 92,582 shares, respectively. For the 2018 Awards, the performance shares earned were 71,074.