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, 2020		71,100,000
Balance at Dec. 31, 2020	$ 3,395.2	$ 0.7	$ 2,148.7	$ (14.2)	$ (2,579.2)	$ 3,839.2
Increase (Decrease) in Stockholders' Equity
Net income	363.6					363.6
Unrealized losses on available-for-sale securities	(6.5)			(6.5)
Defined benefit pension plan	0.6			0.6
Shares issued under employee stock purchase plan (ESPP) (in shares)		100,000
Shares issued under employee stock purchase plan (ESPP)	5.6		5.6
Restricted stock units (RSUs) withheld for taxes	(10.6)		(10.6)
Common stock issued for RSUs vested (in shares)		100,000
Exercise of stock options (in shares)		300,000
Exercise of stock options	42.3		42.3
Share-based compensation	39.5		39.5
Balance (in shares) at Sep. 30, 2021		71,600,000
Balance at Sep. 30, 2021	3,829.7	$ 0.7	2,225.5	(20.1)	(2,579.2)	4,202.8
Balance (in shares) at Jun. 30, 2021		71,500,000
Balance at Jun. 30, 2021	3,639.5	$ 0.7	2,196.5	(18.6)	(2,579.2)	4,040.1
Increase (Decrease) in Stockholders' Equity
Net income	162.7					162.7
Unrealized losses on available-for-sale securities	(1.6)			(1.6)
Defined benefit pension plan	0.1			0.1
Shares issued under employee stock purchase plan (ESPP)	2.8		2.8
Restricted stock units (RSUs) withheld for taxes	(0.2)		(0.2)
Exercise of stock options (in shares)		100,000
Exercise of stock options	14.0		14.0
Share-based compensation	12.4		12.4
Balance (in shares) at Sep. 30, 2021		71,600,000
Balance at Sep. 30, 2021	$ 3,829.7	$ 0.7	2,225.5	(20.1)	(2,579.2)	4,202.8
Balance (in shares) at Dec. 31, 2021	71,727,021	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 losses on available-for-sale securities	(61.8)			(61.8)
Defined benefit pension plan	0.7			0.7
Shares issued under employee stock purchase plan (ESPP) (in shares)		100,000
Shares issued under employee stock purchase plan (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	72,182,311	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 losses on available-for-sale securities	(18.8)			(18.8)
Defined benefit pension plan	0.2			0.2
Shares issued under employee stock purchase plan (ESPP) (in shares)		100,000
Shares issued under employee stock purchase plan (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	72,182,311	72,200,000
Balance at Sep. 30, 2022	$ 4,562.2	$ 0.7	$ 2,314.6	$ (84.1)	$ (2,579.2)	$ 4,910.2