Consolidated Statements of Shareholders' Equity - USD ($) shares in Millions, $ in Millions	Common Stock	Additional Paid In Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings	Total
Balance at the beginning of the period at Dec. 31, 2020	$ 32.4	$ 321.5	$ (50.8)	$ 658.1	$ 961.2
Balance (in shares) at Dec. 31, 2020	43.2
Increase (Decrease) in Shareholders' Equity
Net income				927.4	927.4
Other comprehensive income (loss), net of tax			19.9		19.9
Share-based compensation		19.3			19.3
Shares issued, net of shares withheld for employee taxes	$ 0.2	(14.7)		0.1	(14.4)
Shares issued, net of shares withheld for employee taxes (in shares)	0.3
Share repurchase	$ (1.9)	(12.0)		(186.2)	$ (200.1)
Share repurchase (in shares)	(2.5)				(2.5)
Dividends				(45.9)	$ (45.9)
Balance at the end 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)				(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				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)				(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