Consolidated Statements of Stockholders’ Equity (Deficit) - USD ($) shares in Millions, $ in Millions	Total	Cumulative Effect, Period of Adoption, Adjustment	Common stock	Paid-in capital	Retained (deficit) earnings	Retained (deficit) earnings Cumulative Effect, Period of Adoption, Adjustment	Accumulated other comprehensive income (loss)
Common stock outstanding, beginning balance (in shares) at Sep. 30, 2020			185.1
Balance at beginning of period at Sep. 30, 2020	$ (76.0)		$ 1.9	$ 24.5	$ (110.6)		$ 8.2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	420.3				420.3
Dividends paid	(90.9)			0.6	(91.5)
Stock-based compensation, net of issuances	10.1			10.1
Repurchase of common stock (in shares)			(4.8)
Repurchases of common stock	(127.0)		$ (0.1)		(126.9)
Other comprehensive loss, net of tax	(0.7)						(0.7)
Common stock outstanding, ending balance (in shares) at Sep. 30, 2021			180.3
Balance at end of period at Sep. 30, 2021	134.5	$ (1.3)	$ 1.8	35.2	90.0	$ (1.3)	7.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	424.3				424.3
Dividends paid	(89.2)			0.5	(89.7)
Stock-based compensation, net of issuances (in shares)			0.3
Stock-based compensation, net of issuances	8.4			8.4
Repurchase of common stock (in shares)			(4.5)
Repurchases of common stock	(142.6)				(142.6)
Other comprehensive loss, net of tax	$ (28.8)						(28.8)
Common stock outstanding, ending balance (in shares) at Sep. 30, 2022	176.1		176.1
Balance at end 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	1,419.7				1,419.7
Dividends paid	(21.8)			0.1	(21.9)
Stock-based compensation, net of issuances (in shares)			0.5
Stock-based compensation, net of issuances	3.8			3.8
Repurchase of common stock (in shares)			(41.8)
Repurchases of common stock	(1,539.6)		$ (0.5)		(1,539.1)
Other comprehensive loss, net of tax	$ 34.5						34.5
Common stock outstanding, ending balance (in shares) at Sep. 30, 2023	134.8		134.8
Balance at end of period at Sep. 30, 2023	$ 203.2		$ 1.3	$ 48.0	$ 140.7		$ 13.2