Consolidated Statements of Shareholders' Equity - USD ($) shares in Millions, $ in Millions	Common Stock	Additional Paid In Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings Adjustment SSAT	Retained Earnings	Adjustment SSAT	Total
Balance at the beginning of the period at Dec. 31, 2018	$ 32.0	$ 297.8	$ (34.5)		$ 460.0		$ 755.3
Balance (in shares) at Dec. 31, 2018	42.7
Increase (Decrease) in Shareholders' Equity
Net income					82.7		82.7
Adoption of new lease accounting standard					4.4		4.4
Other comprehensive income (loss), net of tax			(2.4)				(2.4)
Share-based compensation		11.3					11.3
Shares issued, net of shares withheld for employee taxes	$ 0.2	(2.9)					(2.7)
Shares issued, net of shares withheld for employee taxes (in shares)	0.2
Dividends					(37.3)		(37.3)
Balance at the end of the period at Dec. 31, 2019	$ 32.2	306.2	(36.9)	$ (5.6)	504.2	$ (5.6)	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						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
Shares repurchased	$ (1.9)	(12.0)			(186.2)		(200.1)
Shares repurchased (in shares)	(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