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, 2021	$ 30.7	$ 314.1	$ (30.9)	$ 1,353.5	$ 1,667.4
Balance (in shares) at Dec. 31, 2021	41.0
Increase (Decrease) in Shareholders' Equity
Net income				339.2	339.2
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	$ 0.2	(19.5)			(19.3)
Shares issued, net of shares withheld for employee taxes (in shares)	0.2
Shares repurchased	$ (0.5)	(3.1)		(65.0)	(68.6)
Shares repurchased (in shares)	(0.7)
Dividends				(12.9)	(12.9)
Balance at the end of the period at Mar. 31, 2022	$ 30.4	296.2	(30.7)	1,614.8	1,910.7
Balance (in shares) at Mar. 31, 2022	40.5
Balance at the beginning 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
Increase (Decrease) in Shareholders' Equity
Net income					719.9
Other comprehensive income (loss), net of tax					(0.6)
Balance at the end of the period at Jun. 30, 2022	$ 29.2	294.7	(31.5)	1,841.2	2,133.6
Balance (in shares) at Jun. 30, 2022	39.0
Balance at the beginning of the period at Mar. 31, 2022	$ 30.4	296.2	(30.7)	1,614.8	1,910.7
Balance (in shares) at Mar. 31, 2022	40.5
Increase (Decrease) in Shareholders' Equity
Net income				380.7	380.7
Other comprehensive income (loss), net of tax			(0.8)		(0.8)
Share-based compensation		5.7			5.7
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
Shares repurchased	$ (1.2)	(7.0)		(129.9)	(138.1)
Shares repurchased (in shares)	(1.6)
Dividends				(24.4)	(24.4)
Balance at the end of the period at Jun. 30, 2022	$ 29.2	294.7	(31.5)	1,841.2	2,133.6
Balance (in shares) at Jun. 30, 2022	39.0
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					114.8
Other comprehensive income (loss), net of tax					(1.0)
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
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)				(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