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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, 2018
Dec. 31, 2017
Dec. 31, 2016
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]      
Weighted average common shares outstanding - basic (in shares) 24.4 25.1 25.4
Weighted average common shares outstanding - diluted (in shares) 24.5 25.1 25.5
Net income per share - basic (in dollars per share) $ 10.91 $ 9.78 $ 7.76
Net income per share - diluted (in dollars per share) $ 10.86 $ 9.75 $ 7.72
Class A common stock      
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]      
Net income applicable to common stockholders - basic $ 254.8 $ 233.4 $ 182.4
Reallocation of distributed net income as a result of conversion of dilutive stock options 0.0 0.0 0.0
Reallocation of distributed net income due to conversion of Class B to Class A common shares outstanding 1.1 1.3 1.8
Conversion of Class B common shares into Class A common shares 9.8 10.5 12.8
Effect of dilutive stock options on net income 0.0 0.0 0.1
Net income applicable to common stockholders - diluted $ 265.7 $ 245.2 $ 197.1
Weighted average common shares outstanding - basic (in shares) 23.4 23.9 23.5
Conversion of Class B common shares into Class A common shares (in shares) 1.0 1.2 1.9
Effect employee stock purchases and restricted stock units on weighted average common shares (in shares) 0.1 0.0 0.1
Weighted average common shares outstanding - diluted (in shares) 24.5 25.1 25.5
Net income per share - basic (in dollars per share) $ 10.91 $ 9.78 $ 7.76
Net income per share - diluted (in dollars per share) $ 10.86 $ 9.75 $ 7.72
Class A common stock | Employee Stock Option      
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]      
Shares issuable pursuant to employee stock purchases and restricted stock units not included since they were antidilutive (in shares) 0.0 0.0 0.0
Class B common stock      
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]      
Net income applicable to common stockholders - basic $ 10.9 $ 11.8 $ 14.7
Reallocation of distributed net income as a result of conversion of dilutive stock options 0.0 0.0 0.0
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
Effect of dilutive stock options on net income 0.0 0.0 (0.1)
Net income applicable to common stockholders - diluted $ 10.9 $ 11.8 $ 14.6
Weighted average common shares outstanding - basic (in shares) 1.0 1.2 1.9
Conversion of Class B common shares into Class A common shares (in shares) 0.0 0.0 0.0
Effect employee stock purchases and restricted stock units on weighted average common shares (in shares) 0.0 0.0 0.0
Weighted average common shares outstanding - diluted (in shares) 1.0 1.2 1.9
Net income per share - basic (in dollars per share) $ 10.91 $ 9.78 $ 7.76
Net income per share - diluted (in dollars per share) $ 10.86 $ 9.76 $ 7.72
Class B common stock | Employee Stock Option      
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]      
Shares issuable pursuant to employee stock purchases and restricted stock units not included since they were antidilutive (in shares) 0.0 0.0 0.0