CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - 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	$ 3,754.4	$ 30.0	$ 794.9	$ (591.3)		$ 3,520.8
Beginning Balance, Shares at Dec. 31, 2020		283,004.0
Net income	587.1					587.1
Net unrealized holding (loss) gain on available-for-sale securities	(0.6)		(0.5)		$ (0.1)
Foreign currency translation	(9.3)				(9.3)
Employee stock purchase plan	42.9	$ 0.1	42.8
Employee stock purchase plan, Shares		851.0
Stock incentive plans	51.2	$ 0.1	51.1
Stock incentive plans, Shares		1,313.0
Agency acquisition	9.9		9.9
Agency acquisition, Shares		184.0
Directors	0.9		0.9
Directors, Shares		17.0
Repurchase shares to fund tax withholdings for non-cash stock-based compensation	(49.8)	$ (0.1)	(49.7)
Repurchase shares to fund tax withholdings for non-cash stock-based compensation, shares		(1,061.0)
Purchase of treasury stock	(82.6)			(82.6)
Purchase of treasury stock, Shares		(1,812.0)
Cash dividends paid	(107.2)					(107.2)
Ending Balance, Value at Dec. 31, 2021	4,196.9	$ 30.1	849.4	(673.9)	(9.4)	4,000.7
Ending Balance, Shares at Dec. 31, 2021		282,496.0
Net income	671.8					671.8
Net unrealized holding (loss) gain on available-for-sale securities	(1.5)				(1.5)
Foreign currency translation	(136.5)		1.0		(137.5)
Employee stock purchase plan	47.0	$ 0.1	46.9
Employee stock purchase plan, Shares		792.0
Stock incentive plans	55.7	$ 0.2	55.5
Stock incentive plans, Shares		1,588.0
Agency acquisition	14.7		14.7
Agency acquisition, Shares		253.0
Directors	0.9		0.9
Directors, Shares		15.0
Repurchase shares to fund tax withholdings for non-cash stock-based compensation	(48.8)	$ (0.1)	(48.7)
Repurchase shares to fund tax withholdings for non-cash stock-based compensation, shares		(759.0)
Purchase of treasury stock	(74.1)			(74.1)
Purchase of treasury stock, Shares		(1,164.0)
Cash dividends paid	(119.5)					(119.5)
Ending Balance, Value at Dec. 31, 2022	$ 4,606.6	$ 30.3	919.7	(748.0)	148.4	4,553.0
Ending Balance, Shares at Dec. 31, 2022	283.2	283,221.0
Net income	$ 870.5					870.5
Net unrealized holding (loss) gain on available-for-sale securities	0.7				0.7
Foreign currency translation	128.4		(0.2)		128.6
Employee stock purchase plan	52.5	$ 0.1	52.4
Employee stock purchase plan, Shares		744.0
Stock incentive plans	75.6	$ 0.1	75.5
Stock incentive plans, Shares		1,034.0
Agency acquisition	18.3		18.3
Agency acquisition, Shares		262.0
Directors	1.1		1.1
Directors, Shares		16.0
Repurchase shares to fund tax withholdings for non-cash stock-based compensation	(39.8)	$ (0.1)	(39.7)
Repurchase shares to fund tax withholdings for non-cash stock-based compensation, shares		(696.0)
Purchase of treasury stock	(0.1)			(0.1)
Purchase of treasury stock, Shares		(2.0)
Cash dividends paid	(135.0)					(135.0)
Ending Balance, Value at Dec. 31, 2023	$ 5,578.8	$ 30.4	$ 1,027.1	$ (748.1)	$ (19.1)	$ 5,288.5
Ending Balance, Shares at Dec. 31, 2023	284.6	284,579.0