Debt (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	May 22, 2020	Sep. 30, 2019
Debt Instrument [Line Items]
Total long-term debt, including current maturities		$ 1,250.0		$ 1,250.0			$ 1,217.0
Current maturities of long-term debt		0.0		0.0			117.0
Unamortized discount and debit issuance costs	[1],[2]	12.6		12.6			2.2
Long-term debt		1,237.4		1,237.4			1,097.8
Notes payable		18.4		18.4			14.4
Cost of early retirement of long-term debt		26.2	$ 0.0	26.2	$ 0.0
Premium for extinguishment on debt				25.7
Senior notes | Senior notes due 2021
Debt Instrument [Line Items]
Total long-term debt, including current maturities		0.0		0.0			600.0
Unamortized debt issuance costs							0.8
Write off of Deferred Debt Issuance Cost				0.5
Senior Notes		600.0		600.0
Senior notes | Senior notes due 2022
Debt Instrument [Line Items]
Total long-term debt, including current maturities		500.0		500.0			500.0
Unamortized debt issuance costs		0.7		0.7			1.0
Unamortized discount		0.3		0.3			0.4
Senior notes | Senior Notes, Due 2028
Debt Instrument [Line Items]
Total long-term debt, including current maturities		750.0		750.0			0.0
Unamortized debt issuance costs		$ 11.6		$ 11.6		$ 11.7
Long term debt, stated interest rate		5.50%		5.50%
Domestic line of credit
Debt Instrument [Line Items]
Total long-term debt, including current maturities		$ 0.0		$ 0.0			117.0
Revolving Credit Facility due April 2025
Debt Instrument [Line Items]
Letters of credit available		70.0		70.0
Revolving credit facility, borrowing limit		425.0		425.0
Domestic Line Of Credit Member due 2025
Debt Instrument [Line Items]
Total long-term debt, including current maturities		$ 0.0		$ 0.0			$ 0.0

[1] At June 30, 2020 and September 30, 2019, the balance for the Senior Notes due 2022 was reflected net of discount of $0.3 and $0.4, respectively.

[2] At June 30, 2020, the balance for the Senior Notes due 2022 and the Senior Notes due 2028 are reflected net of debt issuance costs of $0.7 and $11.6, respectively. At September 30, 2019, the balance for the Senior Notes due 2021 and the Senior Notes due 2022 are reflected net of debt issuance costs of $0.8 and $1.0, respectively.