Condensed Consolidated Statements of Shareholders' Equity - USD ($) shares in Millions, $ in Millions	Common Stock	Additional Paid In Capital	AOCI Attributable to Parent [Member]	Retained Earnings	Total
Balance at the beginning 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
Increase (Decrease) in Shareholders' Equity
Net income				34.0	34.0
Other comprehensive income (loss), net of tax			0.6		0.6
Share-based compensation		4.6			4.6
Shares issued, net of shares withheld for employee taxes	$ 0.2	(12.6)			(12.4)
Shares issued, net of shares withheld for employee taxes (in shares)	0.3
Shares repurchased	$ (0.5)	(2.7)		(38.9)	(42.1)
Shares repurchased (in shares)	(0.7)
Dividends				(11.3)	(11.3)
Balance at the end of the period at Mar. 31, 2023	$ 26.9	279.7	(6.3)	1,970.0	2,270.3
Balance (in shares) at Mar. 31, 2023	35.9
Balance at the beginning 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
Increase (Decrease) in Shareholders' Equity
Net income					234.7
Other comprehensive income (loss), net of tax					(2.4)
Balance at the end of the period at Sep. 30, 2023	$ 26.2	289.1	(9.3)	2,084.3	2,390.3
Balance (in shares) at Sep. 30, 2023	34.9
Balance at the beginning of the period at Mar. 31, 2023	$ 26.9	279.7	(6.3)	1,970.0	2,270.3
Balance (in shares) at Mar. 31, 2023	35.9
Increase (Decrease) in Shareholders' Equity
Net income				80.8	80.8
Other comprehensive income (loss), net of tax			(1.6)		(1.6)
Share-based compensation		5.2			5.2
Shares issued, net of shares withheld for employee taxes		0.1			0.1
Shares repurchased	$ (0.6)	(2.3)		(39.5)	(42.4)
Shares repurchased (in shares)	(0.6)
Dividends				(22.4)	(22.4)
Balance at the end of the period at Jun. 30, 2023	$ 26.3	282.7	(7.9)	1,988.9	2,290.0
Balance (in shares) at Jun. 30, 2023	35.3
Increase (Decrease) in Shareholders' Equity
Net income				119.9	119.9
Other comprehensive income (loss), net of tax			(1.4)		(1.4)
Share-based compensation		7.8			7.8
Shares issued, net of shares withheld for employee taxes		(0.2)			(0.2)
Shares repurchased	$ (0.1)	(1.2)		(24.5)	(25.8)
Shares repurchased (in shares)	(0.4)
Balance at the end of the period at Sep. 30, 2023	$ 26.2	289.1	(9.3)	2,084.3	2,390.3
Balance (in shares) at Sep. 30, 2023	34.9
Balance at the beginning of the period at Dec. 31, 2023	$ 25.8	293.4	(8.2)	2,089.7	2,400.7
Balance (in shares) at Dec. 31, 2023	34.4
Increase (Decrease) in Shareholders' Equity
Net income				36.1	36.1
Other comprehensive income (loss), net of tax			(1.6)		(1.6)
Share-based compensation		5.7			5.7
Shares issued, net of shares withheld for employee taxes	$ 0.1	(17.3)			(17.2)
Shares issued, net of shares withheld for employee taxes (in shares)	0.2
Shares repurchased	$ (0.3)	(1.6)		(47.0)	(48.9)
Shares repurchased (in shares)	(0.4)
Equity interest in SSAT (see Note 4)				10.1	10.1
Dividends				(11.1)	(11.1)
Balance at the end of the period at Mar. 31, 2024	$ 25.6	280.2	(9.8)	2,077.8	2,373.8
Balance (in shares) at Mar. 31, 2024	34.2
Balance at the beginning of the period at Dec. 31, 2023	$ 25.8	293.4	(8.2)	2,089.7	2,400.7
Balance (in shares) at Dec. 31, 2023	34.4
Increase (Decrease) in Shareholders' Equity
Net income					348.4
Other comprehensive income (loss), net of tax					(1.8)
Shares repurchased					$ (169.2)
Shares repurchased (in shares)					(1.4)
Balance at the end of the period at Sep. 30, 2024	$ 24.9	290.5	(10.0)	2,251.3	$ 2,556.7
Balance (in shares) at Sep. 30, 2024	33.2
Balance at the beginning of the period at Mar. 31, 2024	$ 25.6	280.2	(9.8)	2,077.8	2,373.8
Balance (in shares) at Mar. 31, 2024	34.2
Increase (Decrease) in Shareholders' Equity
Net income				113.2	113.2
Other comprehensive income (loss), net of tax			(0.4)		(0.4)
Share-based compensation		6.3			6.3
Shares issued, net of shares withheld for employee taxes		0.2			0.2
Shares repurchased	$ (0.4)	(2.0)		(69.8)	(72.2)
Shares repurchased (in shares)	(0.6)
Dividends				(22.4)	(22.4)
Balance at the end of the period at Jun. 30, 2024	$ 25.2	284.7	(10.2)	2,098.8	2,398.5
Balance (in shares) at Jun. 30, 2024	33.6
Increase (Decrease) in Shareholders' Equity
Net income				199.1	199.1
Other comprehensive income (loss), net of tax			0.2		0.2
Share-based compensation		7.1			7.1
Shares issued, net of shares withheld for employee taxes		(0.1)			(0.1)
Shares repurchased	$ (0.3)	(1.2)		(46.6)	$ (48.1)
Shares repurchased (in shares)	(0.4)				(0.4)
Balance at the end of the period at Sep. 30, 2024	$ 24.9	$ 290.5	$ (10.0)	$ 2,251.3	$ 2,556.7
Balance (in shares) at Sep. 30, 2024	33.2