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
	Mar. 31, 2020	Mar. 31, 2019
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Weighted average common shares outstanding - basic (in shares)	23.3	23.2
Weighted average common shares outstanding - diluted (in shares)	23.5	23.2
Net income per common share - basic (in dollars per share)	$ 1.99	$ 2.43
Net income per common share - diluted (in dollars per share)	$ 1.97	$ 2.42
Class A common stock
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income applicable to common stockholders - basic	$ 45.0	$ 54.4
Reallocation of net income due to conversion of Class B to Class A common shares outstanding	0.2	0.3
Conversion of Class B common shares into Class A common shares	1.0	1.7
Net income applicable to common stockholders - diluted	$ 46.2	$ 56.4
Weighted average common shares outstanding - basic (in shares)	22.7	22.3
Conversion of Class B common shares into Class A common shares (in shares)	0.6	0.8
Effect of employee stock purchases and restricted stock units on weighted average common shares (in shares)	0.2	0.1
Weighted average common shares outstanding - diluted (in shares)	23.5	23.2
Net income per common share - basic (in dollars per share)	$ 1.99	$ 2.43
Net income per common share - diluted (in dollars per share)	$ 1.97	$ 2.42
Class B common stock
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income applicable to common stockholders - basic	$ 1.2	$ 2.0
Reallocation of net income due to conversion of Class B to Class A common shares outstanding	0.0	0.0
Conversion of Class B common shares into Class A common shares	0.0	0.0
Net income applicable to common stockholders - diluted	$ 1.2	$ 2.0
Weighted average common shares outstanding - basic (in shares)	0.6	0.8
Conversion of Class B common shares into Class A common shares (in shares)	0.0	0.0
Effect of employee stock purchases and restricted stock units on weighted average common shares (in shares)	0.0	0.0
Weighted average common shares outstanding - diluted (in shares)	0.6	0.8
Net income per common share - basic (in dollars per share)	$ 1.99	$ 2.43
Net income per common share - diluted (in dollars per share)	$ 1.97	$ 2.42