Debt (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Sep. 30, 2020
Debt Instrument [Line Items]
Total long-term debt, including current maturities		$ 1,250.0		$ 1,250.0		$ 1,250.0
Unamortized discount and debit issuance costs	[1],[2]	16.4		16.4		12.1
Long-term debt		1,233.6		1,233.6		1,237.9
Notes payable		24.8		24.8		21.1
Cost of early retirement of long-term debt		0.0	$ 26.2	26.1	$ 26.2
Premium for extinguishment on debt				25.5
Senior notes | Senior Notes, Due 2022
Debt Instrument [Line Items]
Total long-term debt, including current maturities		0.0		0.0		500.0
Unamortized discount						$ 0.2
Long term debt, stated interest rate						4.70%
Unamortized debt issuance costs						$ 0.6
Write off of Deferred Debt Issuance Cost				0.6
Senior notes | Senior Notes, Due 2028
Debt Instrument [Line Items]
Total long-term debt, including current maturities		750.0		750.0		750.0
Unamortized debt issuance costs		10.2		10.2		11.3
Senior notes | Senior Notes, Due 2029
Debt Instrument [Line Items]
Total long-term debt, including current maturities		$ 500.0		$ 500.0		$ 0.0
Long term debt, stated interest rate		4.125%		4.125%
Unamortized debt issuance costs		$ 6.2		$ 6.2

[1] At June 30, 2021, the balance for the Senior Notes due 2028 and the Senior Notes due 2029 are reflected net of debt issuance costs of $10.2 and $6.2, respectively. At September 30, 2020, the balance for the Senior Notes due 2022 and the Senior Notes due 2028 are reflected net of debt issuance costs of $0.6 and $11.3, respectively.

[2] At September 30, 2020, the balance for the Senior Notes due 2022 was reflected net of discount of $0.2.