CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) shares in Millions, $ in Millions	Common Stock	Additional Paid In Capital	AOCI Attributable to Parent [Member]	Retained Earnings	Total
Balance at the beginning of the period at Dec. 31, 2021	$ 30.7	$ 314.1	$ (30.9)	$ 1,353.5	$ 1,667.4
Balance (in shares) at Dec. 31, 2021	41.0
Increase (Decrease) in Shareholders' Equity
Net income				1,063.9	1,063.9
Other comprehensive income (loss), net of tax			24.0		24.0
Share-based compensation		18.3			18.3
Shares issued, net of shares withheld for employee taxes	$ 0.2	(20.3)			(20.1)
Shares issued, net of shares withheld for employee taxes (in shares)	0.3
Share repurchase	$ (3.7)	(21.7)		(371.6)	(397.0)
Share repurchase (in shares)	(5.0)
Equity interest in SSAT (see Note 4)				(11.6)	(11.6)
Dividends				(48.0)	(48.0)
Balance at the end of the period at Dec. 31, 2022	$ 27.2	290.4	(6.9)	1,986.2	2,296.9
Balance (in shares) at Dec. 31, 2022	36.3
Increase (Decrease) in Shareholders' Equity
Net income				297.1	297.1
Other comprehensive income (loss), net of tax			(1.3)		(1.3)
Share-based compensation		23.8			23.8
Shares issued, net of shares withheld for employee taxes	$ 0.2	(12.7)		(0.1)	(12.6)
Shares issued, net of shares withheld for employee taxes (in shares)	0.2
Share repurchase	$ (1.6)	(8.1)		(148.5)	(158.2)
Share repurchase (in shares)	(2.1)
Dividends				(45.0)	(45.0)
Balance at the end of the period at Dec. 31, 2023	$ 25.8	293.4	(8.2)	2,089.7	$ 2,400.7
Balance (in shares) at Dec. 31, 2023	34.4				34.4
Increase (Decrease) in Shareholders' Equity
Net income				476.4	$ 476.4
Other comprehensive income (loss), net of tax			1.7		1.7
Share-based compensation		26.5			26.5
Shares issued, net of shares withheld for employee taxes	$ 0.1	(17.7)			(17.6)
Shares issued, net of shares withheld for employee taxes (in shares)	0.2
Share repurchase	$ (1.2)	(5.5)		(194.3)	(201.0)
Share repurchase (in shares)	(1.6)
Equity interest in SSAT (see Note 4)				10.1	10.1
Dividends				(44.8)	(44.8)
Balance at the end of the period at Dec. 31, 2024	$ 24.7	$ 296.7	$ (6.5)	$ 2,337.1	$ 2,652.0
Balance (in shares) at Dec. 31, 2024	33.0				33.0