Debt - Senior Notes (Details) - USD ($)	1 Months Ended		12 Months Ended
	Jan. 31, 2021	Feb. 29, 2020	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Debt Instrument [Line Items]
Proceeds from borrowings, net of issuance costs of $3.0 million and $7.1 million in 2023 and 2021, respectively			$ 921,000,000.0	$ 23,000,000.0	$ 527,900,000
Gain (loss) on extinguishment of debt			$ 0	0	(36,400,000)
2031 Notes | Senior Notes
Debt Instrument [Line Items]
Interest rate of debt			3.625%
Aggregate principal amount			$ 535,000,000	535,000,000.0
Proceeds from borrowings, net of issuance costs of $3.0 million and $7.1 million in 2023 and 2021, respectively	$ 527,500,000
2030 Notes | Senior Notes
Debt Instrument [Line Items]
Interest rate of debt			4.25%
Aggregate principal amount			$ 600,000,000	$ 600,000,000.0
Proceeds from borrowings, net of issuance costs of $3.0 million and $7.1 million in 2023 and 2021, respectively		$ 592,100,000
Senior Unsecured Notes Due 2025
Debt Instrument [Line Items]
Redemption premium	26,300,000
Repayments of senior debt	$ 840,700,000
Gain (loss) on extinguishment of debt					$ 36,400,000
Senior Unsecured Notes Due 2025 | Senior Notes
Debt Instrument [Line Items]
Interest rate of debt	4.375%	5.50%
Aggregate principal amount	$ 800,000,000
Senior Unsecured Notes Due 2024
Debt Instrument [Line Items]
Redemption premium		$ 15,500,000
Repayments of senior debt		393,700,000
Gain (loss) on extinguishment of debt						$ 19,400,000
Senior Unsecured Notes Due 2024 | Senior Notes
Debt Instrument [Line Items]
Aggregate principal amount		375,000,000
Term Loan | Line of Credit | Secured Debt
Debt Instrument [Line Items]
Repayments of senior debt		$ 100,000,000