Consolidated Statements of Changes in Stockholders' Equity - USD ($) shares in Millions, $ in Millions	Total	Common Stock Class A	Common Stock Class B	Additional Paid-In Capital	Accumulated Other Comprehensive Loss	Retained Earnings
Balance (in shares) at Dec. 31, 2016		23.3	1.8
Balance at Dec. 31, 2016	$ 910.8	$ 165.6	$ 0.2	$ 41.2	$ 0.0	$ 703.8
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	245.2					245.2
Issuance of stock in connection with employee stock plans (in shares)		0.1
Issuance of stock in connection with employee stock plans	7.5	$ 7.5
Issuance of restricted stock to employees (in shares)		0.1
Repurchase of Class A common stock (in shares)		(0.4)
Repurchase of Class A common stock	(33.8)	$ (33.8)
Class B common stock converted to Class A common stock (in shares)		0.8	(0.8)
Class B common stock converted to Class A common stock		$ 0.1	$ (0.1)
Compensation for stock and stock option issuances and excess tax benefits from option exercises	11.3	7.6		3.7
Option premiums paid	(33.4)			(33.4)
Dividends paid	(26.5)					(26.5)
Issuance of stock in connection with acquisitions	2.1	$ 2.1
Balance (in shares) at Dec. 31, 2017		23.9	1.0
Balance at Dec. 31, 2017	1,083.2	$ 149.1	$ 0.1	11.3		922.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	265.7					265.7
Issuance of stock in connection with employee stock plans (in shares)		0.1
Issuance of stock in connection with employee stock plans	10.1	$ 10.1
Issuance of restricted stock to employees (in shares)		0.1
Repurchase of Class A common stock (in shares)		(2.1)
Repurchase of Class A common stock	(148.9)	$ (168.5)		19.6
Compensation for stock and stock option issuances and excess tax benefits from option exercises	13.4	$ 9.3		4.1
Dividends paid	(27.7)					(27.7)
Balance (in shares) at Dec. 31, 2018		22.0	1.0
Balance at Dec. 31, 2018	1,197.2		$ 0.1	35.0		1,162.1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	271.5					271.5
Loss on cash flow hedges, net of tax benefit	(0.7)				(0.7)
Issuance of stock in connection with employee stock plans (in shares)		0.1
Issuance of stock in connection with employee stock plans	11.0	$ 11.0
Issuance of restricted stock to employees (in shares)		0.1
Repurchase of Class A common stock	(3.2)	$ (3.2)		0.0
Class B common stock converted to Class A common stock (in shares)		0.4	(0.4)
Compensation for stock and stock option issuances and excess tax benefits from option exercises	16.2	$ 12.7		3.5
Option premiums received (paid)	2.4			7.5		(5.1)
Dividends paid	(27.6)					(27.6)
Balance (in shares) at Dec. 31, 2019		22.6	0.6
Balance at Dec. 31, 2019	$ 1,467.7	$ 20.5	$ 0.1	$ 46.0	$ (0.7)	$ 1,401.8