Consolidated Statements of Stockholders’ Equity (Deficit) - USD ($) shares in Millions, $ in Millions	Total	Common stock	Paid-in capital	Retained earnings	Accumulated other comprehensive income (loss)
Common stock outstanding, beginning balance (in 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	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
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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	211.5			211.5
Stock-based compensation, net of issuances (in shares)		0.4
Stock-based compensation, net of issuances	3.2		3.2
Repurchase of common stock (in shares)		(6.7)
Repurchases of common stock	(229.0)			(229.0)
Other comprehensive loss, net of tax	$ (3.3)				(3.3)
Common stock outstanding, ending balance (in shares) at Sep. 30, 2024	128.5	128.5
Balance at end of period at Sep. 30, 2024	$ 185.6	$ 1.3	$ 51.2	$ 123.2	$ 9.9