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, 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					249.7
Other comprehensive income (loss), net of tax					0.1
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
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
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)
Shares repurchased					$ (206.7)
Shares repurchased (in shares)					(2.3)
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)				(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