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, 2019	$ 32.2	$ 306.2	$ (36.9)	$ 504.2	$ 805.7
Balance (in shares) at Dec. 31, 2019	42.9
Increase (Decrease) in Shareholders' Equity
Net income				193.1	193.1
Other comprehensive income (loss), net of tax			(13.9)		(13.9)
Share-based compensation		18.8			18.8
Shares issued, net of shares withheld for employee taxes	$ 0.2	(3.5)		(2.2)	(5.5)
Shares issued, net of shares withheld for employee taxes (in shares)	0.3
Equity interest in SSAT				2.2	2.2
Dividends				(39.2)	(39.2)
Balance at the end 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				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				(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