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Net Income Per Share of Class A and Class B Common Stock (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]      
Weighted average common shares outstanding – basic 23.8 23.2 24.4
Weighted average common shares outstanding – diluted 24.1 23.4 24.5
Net income per common share - basic (in dollars per share) $ 19.74 $ 11.70 $ 10.91
Net income per common share - diluted (in dollars per share) $ 19.53 $ 11.60 $ 10.86
Class A      
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]      
Net income from continuing operations applicable to common stockholders $ 460.9 $ 264.5 $ 254.8
Reallocation of distributed net income due to conversion of class B to class A common shares outstanding 0.6 0.7 1.1
Conversion of class B common shares into class A common shares 8.9 6.3 9.8
Net income applicable to common stockholders - diluted $ 470.3 $ 271.5 $ 265.7
Weighted average common shares outstanding – basic 23.3 22.6 23.4
Conversion of class B common shares into class A common shares 0.5 0.6 1.0
Effect of employee stock purchases and restricted stock units on weighted average common shares 0.3 0.2 0.1
Weighted average common shares outstanding – diluted 24.1 23.4 24.5
Net income per common share - basic (in dollars per share) $ 19.74 $ 11.70 $ 10.91
Net income per common share - diluted (in dollars per share) $ 19.53 $ 11.60 $ 10.86
Class B      
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]      
Net income from continuing operations applicable to common stockholders $ 9.4 $ 7.0 $ 10.9
Reallocation of distributed net income due to conversion of class B to class A common shares outstanding 0.0 0.0 0.0
Conversion of class B common shares into class A common shares 0.0 0.0 0.0
Net income applicable to common stockholders - diluted $ 9.4 $ 7.0 $ 10.9
Weighted average common shares outstanding – basic 0.5 0.6 1.0
Conversion of class B common shares into class A common shares 0.0 0.0 0.0
Effect of employee stock purchases and restricted stock units on weighted average common shares 0.0 0.0 0.0
Weighted average common shares outstanding – diluted 0.5 0.6 1.0
Net income per common share - basic (in dollars per share) $ 19.74 $ 11.70 $ 10.91
Net income per common share - diluted (in dollars per share) $ 19.53 $ 11.60 $ 10.86