Condensed 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				3.8	3.8
Other comprehensive income (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, 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					107.5
Other comprehensive income (loss), net of tax					(1.0)
Balance at the end of the period at Sep. 30, 2020	$ 32.3	312.6	(37.9)	584.8	891.8
Balance (in shares) at Sep. 30, 2020	43.1
Balance at the beginning 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
Increase (Decrease) in Shareholders' Equity
Net income				32.8	32.8
Other comprehensive income (loss), net of tax			0.1		0.1
Share-based compensation		3.0			3.0
Shares issued, net of shares withheld for employee taxes		(1.0)			(1.0)
Shares issued, net of shares withheld for employee taxes (in shares)	0.1
Dividends				(19.6)	(19.6)
Balance at the end of the period at Jun. 30, 2020	$ 32.3	306.7	(37.4)	513.9	815.5
Balance (in shares) at Jun. 30, 2020	43.1
Increase (Decrease) in Shareholders' Equity
Net income				70.9	70.9
Other comprehensive income (loss), net of tax			(0.5)		(0.5)
Share-based compensation		6.0			6.0
Shares issued, net of shares withheld for employee taxes		(0.1)			(0.1)
Balance at the end of the period at Sep. 30, 2020	$ 32.3	312.6	(37.9)	584.8	891.8
Balance (in shares) at Sep. 30, 2020	43.1
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				87.2	87.2
Other comprehensive income (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
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					532.9
Other comprehensive income (loss), net of tax					(0.1)
Balance at the end of the period at Sep. 30, 2021	$ 31.5	313.9	(50.9)	1,050.3	1,344.8
Balance (in shares) at Sep. 30, 2021	42.0
Balance at the beginning 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
Increase (Decrease) in Shareholders' Equity
Net income				162.5	162.5
Other comprehensive income (loss), net of tax			0.2		0.2
Share-based compensation		4.7			4.7
Shares issued, net of shares withheld for employee taxes		(0.2)			(0.2)
Shares issued, net of shares withheld for employee taxes (in shares)	0.1
Dividends				(23.3)	(23.3)
Balance at the end of the period at Jun. 30, 2021	$ 32.6	316.5	(50.7)	874.4	1,172.8
Balance (in shares) at Jun. 30, 2021	43.5
Increase (Decrease) in Shareholders' Equity
Net income				283.2	283.2
Other comprehensive income (loss), net of tax			(0.2)		(0.2)
Share-based compensation		4.7			4.7
Shares issued, net of shares withheld for employee taxes		(0.1)		0.1
Shares repurchased	$ (1.1)	(7.2)		(107.4)	(115.7)
Shares repurchased (in shares)	(1.5)
Balance at the end of the period at Sep. 30, 2021	$ 31.5	$ 313.9	$ (50.9)	$ 1,050.3	$ 1,344.8
Balance (in shares) at Sep. 30, 2021	42.0