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		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Weighted average common shares outstanding - basic (in shares)	22.9	23.2	23.0	23.2
Weighted average common shares outstanding - diluted (in shares)	23.1	23.4	23.2	23.3
Net income per common share - basic (in dollars per share)	$ 6.95	$ 3.67	$ 12.30	$ 8.77
Net income per common share - diluted (in dollars per share)	$ 6.86	$ 3.64	$ 12.18	$ 8.72
Class A common stock
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income applicable to common stockholders - basic	$ 156.0	$ 83.0	$ 276.4	$ 198.2
Reallocation of net income due to conversion of Class B to Class A common shares outstanding	0.2	0.2	0.5	0.6
Conversion of Class B common shares into Class A common shares	2.6	2.0	5.8	4.7
Net income applicable to common stockholders - diluted	$ 158.8	$ 85.2	$ 282.7	$ 203.5
Weighted average common shares outstanding - basic (in shares)	22.5	22.6	22.5	22.6
Conversion of Class B common shares into Class A common shares (in shares)	0.4	0.6	0.5	0.6
Effect of employee stock purchases and restricted stock units on weighted average common shares (in shares)	0.2	0.2	0.2	0.1
Weighted average common shares outstanding - diluted (in shares)	23.1	23.4	23.2	23.3
Net income per common share - basic (in dollars per share)	$ 6.95	$ 3.67	$ 12.29	$ 8.77
Net income per common share - diluted (in dollars per share)	$ 6.86	$ 3.64	$ 12.18	$ 8.72
Class B common stock
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income applicable to common stockholders - basic	$ 2.8	$ 2.2	$ 6.3	$ 5.3
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	$ 2.8	$ 2.2	$ 6.3	$ 5.3
Weighted average common shares outstanding - basic (in shares)	0.4	0.6	0.5	0.6
Conversion of Class B common shares into Class A common shares (in shares)	0.0	0.0	0.0	0.0
Effect of employee stock purchases and restricted stock units on weighted average common shares (in shares)	0.0	0.0	0.0	0.0
Weighted average common shares outstanding - diluted (in shares)	0.4	0.6	0.5	0.6
Net income per common share - basic (in dollars per share)	$ 6.95	$ 3.67	$ 12.29	$ 8.77
Net income per common share - diluted (in dollars per share)	$ 6.86	$ 3.64	$ 12.18	$ 8.72