Debt (Summary of Long-Term Debt Instruments) (Details) - USD ($) $ in Millions	Sep. 30, 2024	Sep. 30, 2023
Debt Instrument [Line Items]
Long-term debt, principal amount	$ 14,485.5	$ 10,292.5
Less: Unamortized discount and debt issuance costs	304.5	165.7
Less: Fair value hedge accounting adjustments	36.6	80.5
Total Long-term Debt	14,144.4	10,046.3
Less: Current portion of long-term debt	611.4	615.0
Related Party
Debt Instrument [Line Items]
Less: Long-term debt – related party	104.4	150.7
Nonrelated Party
Debt Instrument [Line Items]
Less: Long-term debt – related party	13,428.6	9,280.6
NEOM Green Hydrogen Company | Nonrecourse
Debt Instrument [Line Items]
Long-term debt, principal amount	3,271.8	1,364.8
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 3.35% due 2024
Debt Instrument [Line Items]
Long-term debt, principal amount	$ 0.0	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 4.60% due 2029
Debt Instrument [Line Items]
Long-term debt, principal amount	$ 750.0	0.0
Interest rate, stated percentage	4.60%
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 4.75% due 2031
Debt Instrument [Line Items]
Long-term debt, principal amount	$ 600.0	0.0
Interest rate, stated percentage	4.75%
Senior Notes | Note 4.80% Due 2033
Debt Instrument [Line Items]
Long-term debt, principal amount	$ 600.0	600.0
Interest rate, stated percentage	4.80%
Senior Notes | Note 4.85% due 2034
Debt Instrument [Line Items]
Long-term debt, principal amount	$ 1,150.0	0.0
Interest rate, stated percentage	4.85%
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	3.38%
Bonds | Other variable-rate 7.11% due 2025
Debt Instrument [Line Items]
Long-term debt, principal amount	$ 38.8	41.4
Interest rate, weighted average	7.11%
Bonds | Eurobonds 1.000% due 2025
Debt Instrument [Line Items]
Long-term debt, principal amount	$ 334.0	317.2
Interest rate, stated percentage	1.00%
Bonds | Eurobonds 0.500% due 2028
Debt Instrument [Line Items]
Long-term debt, principal amount	$ 556.7	528.6
Interest rate, stated percentage	0.50%
Bonds | Eurobonds 0.800% Due 2032
Debt Instrument [Line Items]
Long-term debt, principal amount	$ 556.7	528.6
Interest rate, stated percentage	0.80%
Bonds | Eurobonds 4.000% Due 2035
Debt Instrument [Line Items]
Long-term debt, principal amount	$ 779.4	740.1
Interest rate, stated percentage	4.00%
Bonds | Saudi Riyal Loan Facility variable-rate 7.35% due 2027
Debt Instrument [Line Items]
Long-term debt, principal amount	$ 451.1	451.1
Interest rate, variable percentage	7.35%
Bonds | Saudi Riyal Loan Facility 2.0% due 2034
Debt Instrument [Line Items]
Long-term debt, principal amount	$ 222.2	192.1
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	$ 131.5	167.5
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	$ 190.6	186.1
Interest rate, stated percentage	2.66%
Bonds | New Taiwan Dollar variable-rate 2.72% due 2030
Debt Instrument [Line Items]
Long-term debt, principal amount	$ 94.8	3.1
Interest rate, variable percentage	2.72%
Notes Payable, Other Payables
Debt Instrument [Line Items]
Long-term debt, principal amount		0.0
Notes Payable, Other Payables | 4.07% Other
Debt Instrument [Line Items]
Long-term debt, principal amount	$ 9.4
Interest rate, weighted average	4.07%
Related Party | Chinese Renminbi 5.5% due 2025 to 2027 | Related Party
Debt Instrument [Line Items]
Long-term debt, principal amount	$ 279.8	313.5
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	$ 24.6	14.8
Interest rate, stated percentage	5.70%
Finance Lease Obligations | Foreign 10.8% due 2025 to 2036
Debt Instrument [Line Items]
Long-term debt, principal amount	$ 8.0	$ 7.5
Interest rate, weighted average	10.80%