Consolidated Statements of Stockholders' Equity - USD ($) $ in Millions	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Treasury Stock	Retained Earnings
Balance (in shares) at Dec. 31, 2021		71,700,000
Balance at Dec. 31, 2021	$ 3,958.9	$ 0.7	$ 2,245.4	$ (23.0)	$ (2,579.2)	$ 4,315.0
Increase (Decrease) in Stockholders' Equity
Net income	595.2					595.2
Unrealized gains (losses) on available-for-sale securities	(61.8)			(61.8)
Defined benefit pension plan	0.7			0.7
Shares issued under ESPP (in shares)		100,000
Shares issued under ESPP	5.9		5.9
Restricted stock units (RSUs) withheld for taxes	(11.4)		(11.4)
Common stock issued for RSUs vested (in shares)		100,000
Exercise of stock options (in shares)		300,000
Exercise of stock options	32.9		32.9
Share-based compensation	41.8		41.8
Balance (in shares) at Sep. 30, 2022		72,200,000
Balance at Sep. 30, 2022	4,562.2	$ 0.7	2,314.6	(84.1)	(2,579.2)	4,910.2
Balance (in shares) at Jun. 30, 2022		72,100,000
Balance at Jun. 30, 2022	4,318.4	$ 0.7	2,291.5	(65.5)	(2,579.2)	4,670.9
Increase (Decrease) in Stockholders' Equity
Net income	239.3					239.3
Unrealized gains (losses) on available-for-sale securities	(18.8)			(18.8)
Defined benefit pension plan	0.2			0.2
Shares issued under ESPP (in shares)		100,000
Shares issued under ESPP	2.6		2.6
Restricted stock units (RSUs) withheld for taxes	(0.3)		(0.3)
Exercise of stock options	4.7		4.7
Share-based compensation	16.1		16.1
Balance (in shares) at Sep. 30, 2022		72,200,000
Balance at Sep. 30, 2022	$ 4,562.2	$ 0.7	2,314.6	(84.1)	(2,579.2)	4,910.2
Balance (in shares) at Dec. 31, 2022	72,651,280	72,700,000
Balance at Dec. 31, 2022	$ 4,796.7	$ 0.7	2,388.4	(55.5)	(2,579.2)	5,042.3
Increase (Decrease) in Stockholders' Equity
Net income	767.7					767.7
Unrealized gains (losses) on available-for-sale securities	16.7			16.7
Defined benefit pension plan	(4.9)			(4.9)
Shares issued under ESPP	6.6		6.6
Restricted stock units (RSUs) withheld for taxes	(13.7)		(13.7)
Common stock issued for RSUs vested (in shares)		100,000
Exercise of stock options (in shares)		800,000
Exercise of stock options	90.7		90.7
Share-based compensation	$ 52.3		52.3
Balance (in shares) at Sep. 30, 2023	73,600,427	73,600,000
Balance at Sep. 30, 2023	$ 5,712.1	$ 0.7	2,524.3	(43.7)	(2,579.2)	5,810.0
Balance (in shares) at Jun. 30, 2023		73,500,000
Balance at Jun. 30, 2023	5,411.0	$ 0.7	2,497.7	(50.6)	(2,579.2)	5,542.4
Increase (Decrease) in Stockholders' Equity
Net income	267.6					267.6
Unrealized gains (losses) on available-for-sale securities	7.8			7.8
Defined benefit pension plan	(0.9)			(0.9)
Shares issued under ESPP	3.2		3.2
Restricted stock units (RSUs) withheld for taxes	(0.2)		(0.2)
Exercise of stock options (in shares)		100,000
Exercise of stock options	4.6		4.6
Share-based compensation	$ 19.0		19.0
Balance (in shares) at Sep. 30, 2023	73,600,427	73,600,000
Balance at Sep. 30, 2023	$ 5,712.1	$ 0.7	$ 2,524.3	$ (43.7)	$ (2,579.2)	$ 5,810.0