Condensed Consolidated Statements of Stockholders' 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 Stockholders' Equity
Net income				3.8	3.8
Other comprehensive loss, net of tax			(0.6)		(0.6)
Share-based compensation		3.1			3.1
Shares issued, net of shares withheld for employee taxes	$ 0.1	(4.6)			(4.5)
Shares issued, net of shares withheld for employee taxes (in shares)	0.1
Equity interest in SSAT				2.2	2.2
Dividends				(9.5)	(9.5)
Balance at the end of the period at Mar. 31, 2020	$ 32.3	304.7	(37.5)	500.7	800.2
Balance (in shares) at Mar. 31, 2020	43.0
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 Stockholders' Equity
Net income				87.2	87.2
Other comprehensive loss, net of tax			(0.1)		(0.1)
Share-based compensation		4.8			4.8
Shares issued, net of shares withheld for employee taxes	$ 0.2	(14.3)			(14.1)
Shares issued, net of shares withheld for employee taxes (in shares)	0.2
Dividends				(10.1)	(10.1)
Balance at the end of the period at Mar. 31, 2021	$ 32.6	$ 312.0	$ (50.9)	$ 735.2	$ 1,028.9
Balance (in shares) at Mar. 31, 2021	43.4