Debt (Long-term Debt) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Sep. 30, 2020
Debt Instrument [Line Items]
Long-term Debt, Principal Amount	$ 7,684.7	$ 7,950.2
Less: Unamortized discount and debt issuance costs	(50.4)	(55.8)
Less: Fair value hedge accounting adjustments	0.5	5.7
Total Long-term Debt	7,634.8	7,900.1
Less: Current portion of long-term debt	(484.5)	(470.0)
Less: Long-term debt – related party	(274.6)	(297.2)
Long-term Debt	6,875.7	7,132.9
Debentures \| Debenture 8.75% due 2021
Debt Instrument [Line Items]
Long-term Debt, Principal Amount	$ 0.0	18.4
Interest rate, stated percentage	8.75%
Medium-term Notes \| Series E 7.6% due 2026
Debt Instrument [Line Items]
Long-term Debt, Principal Amount	$ 17.2	17.2
Interest rate, weighted average	7.60%
Senior Notes \| Note 3.0% due 2022
Debt Instrument [Line Items]
Long-term Debt, Principal Amount	$ 400.0	400.0
Interest rate, stated percentage	3.00%
Senior Notes \| Note 2.75% due 2023
Debt Instrument [Line Items]
Long-term Debt, Principal Amount	$ 400.0	400.0
Interest rate, stated percentage	2.75%
Senior Notes \| Note 3.35% due 2024
Debt Instrument [Line Items]
Long-term Debt, Principal Amount	$ 400.0	400.0
Interest rate, stated percentage	3.35%
Senior Notes \| Note 1.50% due 2026
Debt Instrument [Line Items]
Long-term Debt, Principal Amount	$ 550.0	550.0
Interest rate, stated percentage	1.50%
Senior Notes \| Note 1.85% due 2027
Debt Instrument [Line Items]
Long-term Debt, Principal Amount	$ 650.0	650.0
Interest rate, stated percentage	1.85%
Senior Notes \| Note 2.05% due 2030
Debt Instrument [Line Items]
Long-term Debt, Principal Amount	$ 900.0	900.0
Interest rate, stated percentage	2.05%
Senior Notes \| Note 2.70% due 2040
Debt Instrument [Line Items]
Long-term Debt, Principal Amount	$ 750.0	750.0
Interest rate, stated percentage	2.70%
Senior Notes \| Note 2.80% due 2050
Debt Instrument [Line Items]
Long-term Debt, Principal Amount	$ 950.0	950.0
Interest rate, stated percentage	2.80%
Bonds \| Variable-rate industrial revenue bonds 0.02% due 2035 to 2050
Debt Instrument [Line Items]
Long-term Debt, Principal Amount	$ 618.9	631.9
Interest rate, weighted average	0.02%
Bonds \| Other 1.57% due 2024 to 2032
Debt Instrument [Line Items]
Interest rate, weighted average	1.57%
Bonds \| Eurobonds 0.375% due 2021
Debt Instrument [Line Items]
Long-term Debt, Principal Amount	$ 0.0	410.3
Interest rate, stated percentage	0.375%
Bonds \| Eurobonds 1.0% due 2025
Debt Instrument [Line Items]
Long-term Debt, Principal Amount	$ 347.4	351.7
Interest rate, stated percentage	1.00%
Bonds \| Eurobonds 0.50% due 2028
Debt Instrument [Line Items]
Long-term Debt, Principal Amount	$ 579.1	586.2
Interest rate, stated percentage	0.50%
Bonds \| Eurobonds 0.80% due 2032
Debt Instrument [Line Items]
Long-term Debt, Principal Amount	$ 579.1	586.2
Interest rate, stated percentage	0.80%
Bonds \| New Taiwan Dollar 1.86% due 2023 to 2028
Debt Instrument [Line Items]
Interest rate, stated percentage	1.86%
Bonds \| Other Foreign due 2023
Debt Instrument [Line Items]
Long-term Debt, Principal Amount	$ 0.3	0.6
Foreign credit facilities \| Other 1.57% due 2024 to 2032
Debt Instrument [Line Items]
Long-term Debt, Principal Amount	14.4	0.0
Foreign credit facilities \| New Taiwan Dollar 1.86% due 2023 to 2028
Debt Instrument [Line Items]
Long-term Debt, Principal Amount	161.8	0.0
Related Party \| Chinese Renminbi 5.5% due 2022 to 2027
Debt Instrument [Line Items]
Long-term Debt, Principal Amount	$ 355.0	338.5
Interest rate, stated percentage	5.50%
Related Party \| Chinese Renminbi 5.7% due 2033
Debt Instrument [Line Items]
Long-term Debt, Principal Amount	$ 3.4	0.0
Interest rate, stated percentage	5.70%
Capital Lease Obligations \| Foreign 11.6% due 2022 to 2036
Debt Instrument [Line Items]
Long-term Debt, Principal Amount	$ 8.1	$ 9.2
Interest rate, weighted average	11.60%

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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Contractual interest rate for funds borrowed, under the debt agreement.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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

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Amount of cumulative increase (decrease) in fair value of hedged liability in fair value hedge, attributable to hedged risk.

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Amount of long-term debt and lease obligation, classified as noncurrent.

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Amount of long-term debt and lease obligation, classified as current.

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Weighted average interest rate of long-term debt outstanding calculated at point in time.

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The amount for notes payable (written promise to pay), payable to related parties, which are due after one year (or one business cycle).

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