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Stock-Based Compensation - Stock Award Information and Fair Value Assumptions (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Stock-based compensation      
Unrecognized compensation cost related to stock options $ 33.5    
Stock-based compensation expense      
Total stock-based compensation cost 32.8 $ 34.1 $ 27.5
Capitalized cost during the period (4.5) (10.6) (1.3)
Amortization of capitalized cost during the period 1.6 7.9 0.8
Total stock-based compensation expense 29.9 31.4 27.0
Income tax benefit $ 9.6 $ 12.9 $ 10.6
Weighted average period of recognition of unrecognized compensation cost related to stock options 1 year 2 months 12 days    
Executive Incentive Program      
Stock-based compensation      
Shares reserved for grant under the plan (in shares) 4,200,000    
Shares available for grant under the plan (in shares) 1,200,000    
Stock options - annual equity grant      
Stock-based compensation      
Stock options granted (in shares) 395,000    
Weighted-average assumptions used to estimate grant date fair value of stock options      
Expected dividend yield (as a percent) 1.30% 1.60%  
Expected life in years 4 years 9 months 18 days 4 years 10 months 24 days  
Expected volatility (as a percent) 21.90% 21.30%  
Risk-free interest rate (as a percent) 1.40% 2.50%  
Weighted average grant date fair value (in dollars per share) $ 29.29 $ 21.07  
Fair value of equity awards granted $ 11.6 $ 9.7  
Stock options, one-time grant      
Weighted-average assumptions used to estimate grant date fair value of stock options      
Expected dividend yield (as a percent)     1.40%
Expected life in years     3 years 10 months 24 days
Expected volatility (as a percent)     20.70%
Risk-free interest rate (as a percent)     2.60%
Weighted average grant date fair value (in dollars per share)     $ 21.91
Fair value of equity awards granted     $ 13.6
Stock option awards      
Stock-based compensation      
Stock options granted (in shares) 395,000    
Unrecognized compensation cost related to stock options $ 6.6    
Stock-based compensation expense      
Total stock-based compensation cost $ 11.2 10.8 $ 11.0
Vesting period 3 years    
Maximum term life 10 years    
Weighted-average assumptions used to estimate grant date fair value of stock options      
Expected dividend yield (as a percent)     1.40%
Expected life in years     5 years 6 months
Expected volatility (as a percent)     23.10%
Risk-free interest rate (as a percent)     2.60%
Weighted average grant date fair value (in dollars per share)     $ 23.71
Fair value of equity awards granted     $ 9.1
Restricted stock awards      
Stock-based compensation      
Awards granted (in shares) 69,000    
Unrecognized compensation cost related to stock options $ 6.6    
Stock-based compensation expense      
Total stock-based compensation cost $ 8.9 7.4 7.7
Vesting period 3 years    
Weighted average period of recognition of unrecognized compensation cost related to stock options 1 year 8 months 12 days    
Performance share awards      
Stock-based compensation      
Awards granted (in shares) 46,000    
Unrecognized compensation cost related to stock options $ 7.7    
Stock-based compensation expense      
Total stock-based compensation cost $ 7.9 $ 6.0 $ 7.4
Vesting period 3 years    
Weighted average period of recognition of unrecognized compensation cost related to stock options 1 year 9 months 18 days    
Restricted stock units      
Stock-based compensation      
Awards granted (in shares) 8,000 12,000 13,000
Stock-based compensation expense      
Total stock-based compensation cost $ 1.4 $ 1.3 $ 1.4
Stock appreciation rights      
Stock-based compensation      
Unrecognized compensation cost related to stock options 2.0    
Stock-based compensation expense      
Total stock-based compensation cost $ 3.4 8.6 $ 0.0
Weighted average period of recognition of unrecognized compensation cost related to stock options 3 months 18 days    
Weighted-average assumptions used to estimate grant date fair value of stock options      
Expected dividend yield (as a percent) 1.50%    
Expected life in years 3 months 18 days    
Expected volatility (as a percent) 30.10%    
Risk-free interest rate (as a percent) 0.10%    
Weighted average grant date fair value (in dollars per share) $ 48.24    
Fair value of equity awards granted $ 13.5    
Q2 2018 Stock option and stock appreciation right grant      
Stock-based compensation expense      
Total stock-based compensation cost 5.1 11.5  
Capitalized cost during the period $ (6.1) $ (3.2)