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Net Income Per Share of Class A and Class B Common Stock (Earnings Per Share Reconciliation) (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]        
Weighted average common shares outstanding - basic (in shares) 23.4 24.8 23.2 24.9
Weighted average common shares outstanding - diluted (in shares) 23.5 24.9 23.3 25.0
Net income per common share - basic (in dollars per share) $ 2.65 $ 2.45 $ 5.10 $ 4.52
Net income per common share - diluted (in dollars per share) $ 2.63 $ 2.44 $ 5.08 $ 4.50
Class A common stock        
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]        
Net income applicable to common stockholders - basic $ 59.8 $ 58.2 $ 114.2 $ 108.2
Reallocation of net income due to conversion of Class B to Class A common shares outstanding 0.2 0.3 0.5 0.6
Conversion of Class B common shares into Class A common shares 1.9 2.2 3.6 3.9
Net income applicable to common stockholders - diluted $ 61.9 $ 60.7 $ 118.3 $ 112.7
Weighted average common shares outstanding - basic (in shares) 22.6 23.8 22.4 23.9
Conversion of Class B common shares into Class A common shares (in shares) 0.8 1.0 0.8 1.0
Effect of dilutive stock options on weighted average common shares (in shares) 0.1 0.1 0.1 0.1
Weighted average common shares outstanding - diluted (in shares) 23.5 24.9 23.3 25.0
Net income per common share - basic (in dollars per share) $ 2.65 $ 2.45 $ 5.10 $ 4.52
Net income per common share - diluted (in dollars per share) $ 2.63 $ 2.44 $ 5.08 $ 4.50
Class B common stock        
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]        
Net income applicable to common stockholders - basic $ 2.1 $ 2.5 $ 4.1 $ 4.5
Reallocation of net income due to conversion of Class B to Class A common shares outstanding 0.0 0.0 0.0 0.0
Conversion of Class B common shares into Class A common shares 0.0 0.0 0.0 0.0
Net income applicable to common stockholders - diluted $ 2.1 $ 2.5 $ 4.1 $ 4.5
Weighted average common shares outstanding - basic (in shares) 0.8 1.0 0.8 1.0
Conversion of Class B common shares into Class A common shares (in shares) 0.0 0.0 0.0 0.0
Effect of dilutive stock options on weighted average common shares (in shares) 0.0 0.0 0.0 0.0
Weighted average common shares outstanding - diluted (in shares) 0.8 1.0 0.8 1.0
Net income per common share - basic (in dollars per share) $ 2.65 $ 2.45 $ 5.10 $ 4.52
Net income per common share - diluted (in dollars per share) $ 2.63 $ 2.44 $ 5.08 $ 4.50