CONDENSED CONSOLIDATED STATEMENTS OF EQUITY (UNAUDITED) - USD ($) shares in Millions, $ in Millions	Total	Common Stock	Additional Paid-in Capital	Treasury Stock	Accumulated Other Comprehensive Loss	Retained Earnings
Beginning Balance, Value at Dec. 31, 2020	$ 3,754.2	$ 30.0	$ 794.9	$ (591.4)		$ 3,520.7
Beginning Balance, Shares at Dec. 31, 2020		283.0
Net income	199.7					199.7
Net unrealized holding (loss) gain on available-for- sale securities	(0.2)		(0.5)		$ 0.3
Foreign currency translation	(5.1)				(5.3)	0.2
Employee stock purchase plan	3.0		3.0
Stock incentive plans	15.6	$ 0.1	15.5
Stock incentive plans, Shares		1.4
Agency acquisition	4.9		4.9
Agency acquisition, Shares		0.1
Repurchase shares to fund tax withholdings for non-cash stock-based compensation	(45.0)	$ (0.1)	(44.9)
Repurchase shares to fund tax withholdings for non-cash stock-based compensation, shares		(1.0)
Purchase of treasury stock	(70.0)			(70.0)
Purchase of treasury stock, Shares		(1.5)
Cash dividends paid	(26.1)					(26.1)
Ending Balance, Value at Mar. 31, 2021	3,831.0	$ 30.0	772.9	(661.4)	(5.0)	3,694.5
Ending Balance, Shares at Mar. 31, 2021		282.0
Beginning Balance, Value at Dec. 31, 2021	$ 4,196.9	$ 30.1	849.4	(673.9)	(9.4)	4,000.7
Beginning Balance, Shares at Dec. 31, 2021	282.5	282.5
Net income	$ 220.3					220.3
Net unrealized holding (loss) gain on available-for- sale securities	(0.9)				(0.9)
Foreign currency translation	(2.1)				(2.1)
Employee stock purchase plan	2.7		2.7
Stock incentive plans	17.5	$ 0.2	17.3
Stock incentive plans, Shares		1.7
Agency acquisition	0.0
Repurchase shares to fund tax withholdings for non-cash stock-based compensation	(46.0)	$ (0.1)	(45.9)
Repurchase shares to fund tax withholdings for non-cash stock-based compensation, shares		(0.7)
Purchase of treasury stock	(24.1)			(24.1)
Purchase of treasury stock, Shares		(0.4)
Cash dividends paid	(28.9)					(28.9)
Ending Balance, Value at Mar. 31, 2022	$ 4,335.4	$ 30.2	$ 823.5	$ (698.0)	$ (12.4)	$ 4,192.1
Ending Balance, Shares at Mar. 31, 2022	283.1	283.1