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, 2019	Dec. 31, 2018	Dec. 31, 2017
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Weighted average common shares outstanding – basic	23.2	24.4	25.1
Weighted average common shares outstanding – diluted	23.4	24.5	25.1
Net income per common share - basic (in dollars per share)	$ 11.70	$ 10.91	$ 9.78
Net income per common share - diluted (in dollars per share)	$ 11.60	$ 10.86	$ 9.75
Class A common stock
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income from continuing operations applicable to common stockholders	$ 264.5	$ 254.8	$ 233.4
Reallocation of distributed net income due to conversion of Class B to Class A common shares outstanding	0.7	1.1	1.3
Conversion of Class B common shares into Class A common shares	6.3	9.8	10.5
Net income applicable to common stockholders - diluted	$ 271.5	$ 265.7	$ 245.2
Weighted average common shares outstanding – basic	22.6	23.4	23.9
Conversion of Class B common shares into Class A common shares	0.6	1.0	1.2
Effect of employee stock purchases and restricted stock units on weighted average common shares	0.2	0.1	0.0
Weighted average common shares outstanding – diluted	23.4	24.5	25.1
Net income per common share - basic (in dollars per share)	$ 11.70	$ 10.91	$ 9.78
Net income per common share - diluted (in dollars per share)	$ 11.60	$ 10.86	$ 9.75
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 not included since they were antidilutive	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 from continuing operations applicable to common stockholders	$ 7.0	$ 10.9	$ 11.8
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	$ 7.0	$ 10.9	$ 11.8
Weighted average common shares outstanding – basic	0.6	1.0	1.2
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	0.6	1.0	1.2
Net income per common share - basic (in dollars per share)	$ 11.70	$ 10.91	$ 9.78
Net income per common share - diluted (in dollars per share)	$ 11.60	$ 10.86	$ 9.75
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 not included since they were antidilutive	0.0	0.0	0.0