Debt (Summary of Long-Term Debt Instruments) (Details) - USD ($) $ in Millions	Sep. 30, 2023	Sep. 30, 2022
Debt Instrument [Line Items]
Long-term debt, Principal Amount	$ 10,292.5	$ 7,309.3
Less: Unamortized discount and debt issuance costs	(165.7)	(45.4)
Less: Fair value hedge accounting adjustments	(80.5)	(77.1)
Total Long-term Debt	10,046.3	7,634.1
Less: Current portion of long-term debt	(615.0)	(548.3)
Related Party
Debt Instrument [Line Items]
Long-term debt	150.7	652.0
Nonrelated Party
Debt Instrument [Line Items]
Long-term debt	9,280.6	6,433.8
NEOM Green Hydrogen Company | Nonrecourse
Debt Instrument [Line Items]
Long-term debt, Principal Amount	1,364.8	0.0
Variable Interest Entity, Primary Beneficiary | NEOM Green Hydrogen Company | Related Party
Debt Instrument [Line Items]
Long-term debt	0.0	447.3
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 2.75% due 2023
Debt Instrument [Line Items]
Long-term debt, Principal Amount	$ 0.0	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 4.80% Due 2033
Debt Instrument [Line Items]
Long-term debt, Principal Amount	$ 600.0	0.0
Interest rate, stated percentage	4.80%
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 3.55% due 2035 to 2050
Debt Instrument [Line Items]
Long-term debt, Principal Amount	$ 618.9	618.9
Interest rate, weighted average	3.55%
Bonds | Other variable-rate 6.78% due 2024 to 2032
Debt Instrument [Line Items]
Long-term debt, Principal Amount	$ 41.4	41.4
Interest rate, weighted average	6.78%
Bonds | Eurobonds 1.000% due 2025
Debt Instrument [Line Items]
Long-term debt, Principal Amount	$ 317.2	294.1
Interest rate, stated percentage	1.00%
Bonds | Eurobonds 0.500% due 2028
Debt Instrument [Line Items]
Long-term debt, Principal Amount	$ 528.6	490.1
Interest rate, stated percentage	0.50%
Bonds | Eurobonds 0.800% Due 2032
Debt Instrument [Line Items]
Long-term debt, Principal Amount	$ 528.6	490.1
Interest rate, stated percentage	0.80%
Bonds | Eurobonds 4.000% Due 2035
Debt Instrument [Line Items]
Long-term debt, Principal Amount	$ 740.1	0.0
Interest rate, stated percentage	4.00%
Bonds | Saudi Riyal Loan Facility variable-rate 4.10% due 2023
Debt Instrument [Line Items]
Long-term debt, Principal Amount	$ 0.0	195.6
Interest rate, variable percentage	4.10%
Bonds | Saudi Riyal Loan Facility variable-rate 7.00% due 2027
Debt Instrument [Line Items]
Long-term debt, Principal Amount	$ 451.1	0.0
Interest rate, variable percentage	7.00%
Bonds | Saudi Riyal Loan Facility 2.0% due 2034
Debt Instrument [Line Items]
Long-term debt, Principal Amount	$ 192.1	0.0
Interest rate, stated percentage	2.00%
Bonds | New Taiwan Dollar 1.86% due 2024 to 2028
Debt Instrument [Line Items]
Long-term debt, Principal Amount	$ 167.5	189.0
Interest rate, stated percentage	1.86%
Bonds | New Taiwan Dollar 2.66% due 2026 to 2029
Debt Instrument [Line Items]
Long-term debt, Principal Amount	$ 186.1	31.5
Interest rate, stated percentage	2.66%
Bonds | New Taiwan Dollar variable-rate 2.55% due 2026 to 2030
Debt Instrument [Line Items]
Long-term debt, Principal Amount	$ 3.1	0.0
Interest rate, variable percentage	2.55%
Bonds | Other payable in other currencies due 2023
Debt Instrument [Line Items]
Long-term debt, Principal Amount	$ 0.0	0.1
Related Party | Chinese Renminbi 5.5% due 2024 to 2027 | Related Party
Debt Instrument [Line Items]
Long-term debt, Principal Amount	$ 313.5	321.5
Interest rate, stated percentage	5.50%
Related Party | Chinese Renminbi 5.7% due 2033 | Related Party
Debt Instrument [Line Items]
Long-term debt, Principal Amount	$ 14.8	12.2
Interest rate, stated percentage	5.70%
Finance Lease Obligations | Foreign 11.4% due 2025 to 2036
Debt Instrument [Line Items]
Long-term debt, Principal Amount	$ 7.5	$ 7.6
Interest rate, weighted average	11.40%