Long-term and Short-term Debt - Summary of Long Term Debt (Details) - USD ($) $ in Millions	Mar. 31, 2020	Sep. 30, 2019
Debt Instrument [Line Items]
Unamortized discount and other	$ (50.8)	$ (50.1)
Total	1,970.2	2,256.9
Less current portion	0.0	(300.5)
Long-term debt	1,970.2	1,956.4
2.050% notes, payable in March 2020
Debt Instrument [Line Items]
Long-term debt, gross	$ 0.0	299.4
Stated interest rate	2.05%
2.875% notes, payable in March 2025
Debt Instrument [Line Items]
Long-term debt, gross	$ 321.0	307.6
Stated interest rate	2.875%
6.70% debentures, payable in January 2028
Debt Instrument [Line Items]
Long-term debt, gross	$ 250.0	250.0
Stated interest rate	6.70%
3.500% notes, payable in March 2029
Debt Instrument [Line Items]
Long-term debt, gross	$ 425.0	425.0
Stated interest rate	3.50%
6.25% debentures, payable in December 2037
Debt Instrument [Line Items]
Long-term debt, gross	$ 250.0	250.0
Stated interest rate	6.25%
4.200% notes, payable in March 2049
Debt Instrument [Line Items]
Long-term debt, gross	$ 575.0	575.0
Stated interest rate	4.20%
5.20% debentures, payable in January 2098
Debt Instrument [Line Items]
Long-term debt, gross	$ 200.0	$ 200.0
Stated interest rate	5.20%

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Amount of unamortized debt discount (premium) and debt issuance costs.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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