Consolidated Statements of Stockholders' 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	Retained Earnings	Adjustment SSAT	Adjustment	Total
Balance at the beginning of the period at Dec. 31, 2017	$ 31.9	$ 289.7	$ (24.9)			$ 380.5			$ 677.2
Balance (in shares) at Dec. 31, 2017	42.5
Increase (Decrease) in Stockholders' Equity
Net income						109.0			109.0
Other comprehensive income (loss), net of tax			(9.6)			6.0			(3.6)
Share-based compensation		12.1							12.1
Shares issued, net of shares withheld for employee taxes	$ 0.1	(4.0)							(3.9)
Shares issued, net of shares withheld for employee taxes (in shares)	0.2
Shares repurchased						(0.1)			(0.1)
Dividends						(35.4)			(35.4)
Balance at the end 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 Stockholders' Equity
Net income						82.7			82.7
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)	$ 4.4	504.2	$ (5.6)	$ 4.4	$ 805.7
Balance (in shares) at Dec. 31, 2019	42.9								42.9
Increase (Decrease) in Stockholders' 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