Long-term and Short-term Debt - Summary of Long Term Debt (Details) - USD ($) $ in Millions	Mar. 31, 2019	Sep. 30, 2018
Debt Instrument [Line Items]
Unamortized discount and other	$ (62.6)	$ (50.8)
Total	2,229.8	1,225.2
Less current portion	(297.4)	0.0
Long-term debt	1,932.4	1,225.2
Notes \| 2.050% notes, payable in March 2020
Debt Instrument [Line Items]
Long-term debt, gross	$ 297.4	294.6
Stated interest rate	2.05%
Notes \| 2.875% notes, payable in March 2025
Debt Instrument [Line Items]
Long-term debt, gross	$ 295.0	281.4
Stated interest rate	2.875%
Notes \| 6.70% debentures, payable in January 2028
Debt Instrument [Line Items]
Long-term debt, gross	$ 250.0	250.0
Stated interest rate	6.70%
Notes \| 3.500% notes, payable in March 2029
Debt Instrument [Line Items]
Long-term debt, gross	$ 425.0	0.0
Stated interest rate	3.50%
Notes \| 6.25% debentures, payable in December 2037
Debt Instrument [Line Items]
Long-term debt, gross	$ 250.0	250.0
Stated interest rate	6.25%
Notes \| 4.200% notes, payable in March 2049
Debt Instrument [Line Items]
Long-term debt, gross	$ 575.0	0.0
Stated interest rate	4.20%
Notes \| 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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