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	239.9					239.9
Unrealized gains (losses) on available-for-sale securities	(31.6)			(31.6)
Defined benefit pension plan	0.3			0.3
Shares issued under employee stock purchase plan (ESPP)	3.3		3.3
Restricted stock units (RSUs) withheld for taxes	(11.1)		(11.1)
Common stock issued for RSUs vested (in shares)		100,000
Exercise of stock options (in shares)		100,000
Exercise of stock options	4.3		4.3
Share-based compensation	12.3		12.3
Balance (in shares) at Mar. 31, 2022		71,900,000
Balance at Mar. 31, 2022	$ 4,176.3	$ 0.7	2,254.2	(54.3)	(2,579.2)	4,554.9
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	240.9					240.9
Unrealized gains (losses) on available-for-sale securities	19.9			19.9
Defined benefit pension plan	(3.2)			(3.2)
Shares issued under employee stock purchase plan (ESPP)	3.4		3.4
Restricted stock units (RSUs) withheld for taxes	(13.5)		(13.5)
Common stock issued for RSUs vested (in shares)		100,000
Exercise of stock options (in shares)		500,000
Exercise of stock options	61.4		61.4
Share-based compensation	$ 17.6		17.6
Balance (in shares) at Mar. 31, 2023	73,317,754	73,300,000
Balance at Mar. 31, 2023	$ 5,123.2	$ 0.7	$ 2,457.3	$ (38.8)	$ (2,579.2)	$ 5,283.2