Condensed Consolidated Statements of Stockholders' Deficit - USD ($) shares in Millions, $ in Millions	Total	Common stock	Paid-in capital	Retained earnings	Accumulated other comprehensive (loss) income
Common stock outstanding, at beginning of period (shares) at Sep. 30, 2021		180.3
Balance at beginning of period at Sep. 30, 2021	$ 134.5	$ 1.8	$ 35.2	$ 90.0	$ 7.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	87.0			87.0
Dividends paid	(22.5)		0.1	(22.6)
Stock-based compensation, net of issuances (in shares)		0.2
Stock-based compensation, net of issuances	(2.4)		(2.4)
Repurchases of common stock (shares)		(0.9)
Repurchases of common stock	(31.4)			(31.4)
Other comprehensive income, net of tax	2.1				2.1
Common stock outstanding, at end of period (shares) at Dec. 31, 2021		179.6
Balance at end of period at Dec. 31, 2021	167.3	$ 1.8	32.9	123.0	9.6
Common stock outstanding, at beginning of period (shares) at Sep. 30, 2021		180.3
Balance at beginning of period at Sep. 30, 2021	134.5	$ 1.8	35.2	90.0	7.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	266.6
Other comprehensive income, net of tax	(10.7)
Common stock outstanding, at end of period (shares) at Jun. 30, 2022		177.4
Balance at end of period at Jun. 30, 2022	225.5	$ 1.8	41.3	185.6	(3.2)
Common stock outstanding, at beginning of period (shares) at Dec. 31, 2021		179.6
Balance at beginning of period at Dec. 31, 2021	167.3	$ 1.8	32.9	123.0	9.6
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	81.4			81.4
Dividends paid	(22.5)		0.1	(22.6)
Stock-based compensation, net of issuances	3.8		3.8
Repurchases of common stock (shares)		(1.0)
Repurchases of common stock	(34.9)			(34.9)
Other comprehensive income, net of tax	5.2				5.2
Common stock outstanding, at end of period (shares) at Mar. 31, 2022		178.6
Balance at end of period at Mar. 31, 2022	200.3	$ 1.8	36.8	146.9	14.8
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	98.2			98.2
Dividends paid	(22.2)		0.2	(22.4)
Stock-based compensation, net of issuances	4.3		4.3
Repurchases of common stock (shares)		(1.2)
Repurchases of common stock	(37.1)			(37.1)
Other comprehensive income, net of tax	(18.0)				(18.0)
Common stock outstanding, at end of period (shares) at Jun. 30, 2022		177.4
Balance at end of period at Jun. 30, 2022	$ 225.5	$ 1.8	41.3	185.6	(3.2)
Common stock outstanding, at beginning of period (shares) at Sep. 30, 2022	176.1	176.1
Balance at beginning of period at Sep. 30, 2022	$ 306.6	$ 1.8	44.1	282.0	(21.3)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	81.9			81.9
Dividends paid	(21.8)		0.1	(21.9)
Stock-based compensation, net of issuances (in shares)		0.3
Stock-based compensation, net of issuances	(3.4)		(3.4)
Repurchases of common stock (shares)		(2.9)
Repurchases of common stock	(87.5)	$ (0.1)		(87.4)
Other comprehensive income, net of tax	13.5				13.5
Common stock outstanding, at end of period (shares) at Dec. 31, 2022		173.5
Balance at end of period at Dec. 31, 2022	$ 289.3	$ 1.7	40.8	254.6	(7.8)
Common stock outstanding, at beginning of period (shares) at Sep. 30, 2022	176.1	176.1
Balance at beginning of period at Sep. 30, 2022	$ 306.6	$ 1.8	44.1	282.0	(21.3)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	1,370.8
Repurchases of common stock (shares)		(35.0)
Repurchases of common stock		$ (1,302.6)
Other comprehensive income, net of tax	$ 38.3
Common stock outstanding, at end of period (shares) at Jun. 30, 2023	138.7	138.7
Balance at end of period at Jun. 30, 2023	$ 285.4	$ 1.4	44.8	222.2	17.0
Common stock outstanding, at beginning of period (shares) at Dec. 31, 2022		173.5
Balance at beginning of period at Dec. 31, 2022	289.3	$ 1.7	40.8	254.6	(7.8)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	1,227.3			1,227.3
Stock-based compensation, net of issuances (in shares)		0.1
Stock-based compensation, net of issuances	1.8		1.8
Repurchases of common stock (shares)		(4.9)
Repurchases of common stock	(171.7)			(171.7)
Other comprehensive income, net of tax	25.7				25.7
Common stock outstanding, at end of period (shares) at Mar. 31, 2023		168.7
Balance at end of period at Mar. 31, 2023	1,372.4	$ 1.7	42.6	1,310.2	17.9
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	61.6			61.6
Stock-based compensation, net of issuances (in shares)		0.1
Stock-based compensation, net of issuances	2.2		2.2
Repurchases of common stock (shares)		(30.1)
Repurchases of common stock	(1,149.9)	$ (0.3)		(1,149.6)
Other comprehensive income, net of tax	$ (0.9)				(0.9)
Common stock outstanding, at end of period (shares) at Jun. 30, 2023	138.7	138.7
Balance at end of period at Jun. 30, 2023	$ 285.4	$ 1.4	$ 44.8	$ 222.2	$ 17.0