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, 2020	3,754.2	$ 30.0	794.9	(591.4)		3,520.7
Beginning Balance, Shares at Dec. 31, 2020		283.0
Net income	485.4
Ending Balance, Value at Sep. 30, 2021	4,107.8	$ 30.1	830.1	(673.9)	(6.5)	3,928.0
Ending Balance, Shares at Sep. 30, 2021		282.5
Beginning Balance, Value at Mar. 31, 2021	3,831.0	$ 30.0	772.9	(661.4)	(5.0)	3,694.5
Beginning Balance, Shares at Mar. 31, 2021		282.0
Net income	139.3					139.3
Foreign currency translation	1.5				1.5
Employee stock purchase plan	2.0		2.0
Stock incentive plans	12.0		12.0
Directors	0.9		0.9
Repurchase shares to fund tax withholdings for non-cash stock-based compensation	(3.2)		(3.2)
Repurchase shares to fund tax withholdings for non-cash stock-based compensation, shares		(0.1)
Purchase of treasury stock	(11.4)			(11.4)
Purchase of treasury stock, Shares		(0.2)
Cash dividends paid	(26.1)					(26.1)
Ending Balance, Value at Jun. 30, 2021	3,946.0	$ 30.0	784.6	(672.8)	(3.5)	3,807.7
Ending Balance, Shares at Jun. 30, 2021		281.7
Net income	146.4					146.4
Net unrealized holding (loss) gain on available-for- sale securities	(0.1)				(0.1)
Foreign currency translation	(2.9)				(2.9)
Employee stock purchase plan	35.3	$ 0.1	35.2
Employee stock purchase plan, Shares		0.8
Stock incentive plans	11.6		11.6
Repurchase shares to fund tax withholdings for non-cash stock-based compensation	(1.3)		(1.3)
Purchase of treasury stock	(1.1)			(1.1)
Cash dividends paid	(26.1)					(26.1)
Ending Balance, Value at Sep. 30, 2021	4,107.8	$ 30.1	830.1	(673.9)	(6.5)	3,928.0
Ending Balance, Shares at Sep. 30, 2021		282.5
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
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
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	$ 526.6
Agency acquisition	14.7
Ending Balance, Value at Sep. 30, 2022	$ 4,310.6	$ 30.3	903.4	(748.0)	(315.5)	4,440.4
Ending Balance, Shares at Sep. 30, 2022	283.3	283.3
Beginning Balance, Value at Mar. 31, 2022	$ 4,335.4	$ 30.2	823.5	(698.0)	(12.4)	4,192.1
Beginning Balance, Shares at Mar. 31, 2022		283.1
Net income	145.2					145.2
Net unrealized holding (loss) gain on available-for- sale securities	(0.3)				(0.3)
Foreign currency translation	(130.2)		(0.1)		(130.1)
Employee stock purchase plan	1.7		1.7
Stock incentive plans	12.1		12.1
Stock incentive plans, Shares		(0.1)
Directors	0.9		0.9
Repurchase shares to fund tax withholdings for non-cash stock-based compensation	(2.4)		(2.4)
Purchase of treasury stock	(50.0)			(50.0)
Purchase of treasury stock, Shares		(0.8)
Cash dividends paid	(29.0)					(29.0)
Ending Balance, Value at Jun. 30, 2022	4,283.4	$ 30.2	835.7	(748.0)	(142.8)	4,308.3
Ending Balance, Shares at Jun. 30, 2022		282.2
Net income	161.1					161.1
Net unrealized holding (loss) gain on available-for- sale securities	(0.4)				(0.4)
Foreign currency translation	(171.7)		0.6		(172.3)
Employee stock purchase plan	39.2	$ 0.1	39.1
Employee stock purchase plan, Shares		0.8
Stock incentive plans	13.6		13.6
Agency acquisition	14.7		14.7
Agency acquisition, Shares		0.3
Repurchase shares to fund tax withholdings for non-cash stock-based compensation	(0.3)		(0.3)
Cash dividends paid	(29.0)					(29.0)
Ending Balance, Value at Sep. 30, 2022	$ 4,310.6	$ 30.3	$ 903.4	$ (748.0)	$ (315.5)	$ 4,440.4
Ending Balance, Shares at Sep. 30, 2022	283.3	283.3