Debt (Summary of Long-Term Debt Instruments) (Details) - USD ($) $ in Millions	2 Months Ended	12 Months Ended
	Nov. 20, 2025	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Debt Instrument [Line Items]
Long-term debt, principal amount		$ 17,996.0	$ 14,485.5
Less: Unamortized discount and debt issuance costs		308.9	304.5
Less: Fair value hedge accounting adjustments		23.4	36.6
Total Long-term Debt		17,663.7	14,144.4
Current portion of long-term debt		716.3	611.4
Repayment of debt		429.9	486.2	$ 615.4
Related Party
Debt Instrument [Line Items]
Current portion of long-term debt		59.0	200.0
Long-term debt		177.5	104.4
Nonrelated Party
Debt Instrument [Line Items]
Long-term debt		16,769.9	13,428.6
NEOM Green Hydrogen Company | Nonrecourse
Debt Instrument [Line Items]
Long-term debt, principal amount		4,909.7	3,271.8
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 1.50% due 2026
Debt Instrument [Line Items]
Long-term debt, principal amount		$ 550.0	550.0
Interest rate		1.50%
Senior Notes | Note 1.50% due 2026 | Subsequent event
Debt Instrument [Line Items]
Repayment of debt	$ 550.0
Senior Notes | Note 1.85% due 2027
Debt Instrument [Line Items]
Long-term debt, principal amount		$ 650.0	650.0
Interest rate		1.85%
Senior Notes | Note 4.3% due 2028
Debt Instrument [Line Items]
Long-term debt, principal amount		$ 600.0	0.0
Interest rate		4.30%
Senior Notes | Note 4.60% due 2029
Debt Instrument [Line Items]
Long-term debt, principal amount		$ 750.0	750.0
Interest rate		4.60%
Senior Notes | Note 2.05% due 2030
Debt Instrument [Line Items]
Long-term debt, principal amount		$ 900.0	900.0
Interest rate		2.05%
Senior Notes | Note 4.75% due 2031
Debt Instrument [Line Items]
Long-term debt, principal amount		$ 600.0	600.0
Interest rate		4.75%
Senior Notes | Note 4.80% due 2033
Debt Instrument [Line Items]
Long-term debt, principal amount		$ 600.0	600.0
Interest rate		4.80%
Senior Notes | Note 4.9% due 2033
Debt Instrument [Line Items]
Long-term debt, principal amount		$ 500.0	0.0
Interest rate		4.90%
Senior Notes | Note 4.85% due 2034
Debt Instrument [Line Items]
Long-term debt, principal amount		$ 1,150.0	1,150.0
Interest rate		4.85%
Senior Notes | Note 2.70% due 2040
Debt Instrument [Line Items]
Long-term debt, principal amount		$ 750.0	750.0
Interest rate		2.70%
Senior Notes | Note 2.80% due 2050
Debt Instrument [Line Items]
Long-term debt, principal amount		$ 950.0	950.0
Interest rate		2.80%
Senior Notes | Green Bonds due 2029 to 2034
Debt Instrument [Line Items]
Debt instrument, face amount		$ 2,500.0
Bonds | Variable-rate industrial revenue bonds 2.802% due 2035 to 2050
Debt Instrument [Line Items]
Long-term debt, principal amount		$ 618.9	618.9
Interest rate, weighted average		2.802%
Bonds | Other variable-rate 7.11% due 2025
Debt Instrument [Line Items]
Long-term debt, principal amount		$ 0.0	38.8
Interest rate, variable percentage		7.11%
Bonds | Eurobonds 1.000% due 2025
Debt Instrument [Line Items]
Long-term debt, principal amount		$ 0.0	334.0
Interest rate		1.00%
Bonds | Eurobonds 0.500% due 2028
Debt Instrument [Line Items]
Long-term debt, principal amount		$ 586.8	556.7
Interest rate		0.50%
Bonds | Eurobonds 2.95% due 2031
Debt Instrument [Line Items]
Long-term debt, principal amount		$ 586.8	0.0
Interest rate		2.95%
Bonds | Eurobonds 0.800% due 2032
Debt Instrument [Line Items]
Long-term debt, principal amount		$ 586.8	556.7
Interest rate		0.80%
Bonds | Eurobonds 3.25% due 2032
Debt Instrument [Line Items]
Long-term debt, principal amount		$ 586.8	0.0
Interest rate		3.25%
Bonds | Eurobonds 4.000% due 2035
Debt Instrument [Line Items]
Long-term debt, principal amount		$ 821.5	779.4
Interest rate		4.00%
Bonds | Eurobonds 3.45% due 2037
Debt Instrument [Line Items]
Long-term debt, principal amount		$ 586.8	0.0
Interest rate		3.45%
Bonds | Saudi Riyal Loan Facility variable-rate 7.35% due 2027
Debt Instrument [Line Items]
Long-term debt, principal amount		$ 0.0	$ 451.1
Interest rate, variable percentage			7.35%
Bonds | Saudi Riyal Loan Facility 2.00% due 2026 to 2034
Debt Instrument [Line Items]
Long-term debt, principal amount		0.0	$ 222.2
Interest rate			2.00%
Bonds | New Taiwan Dollar 1.86% due 2026 to 2028
Debt Instrument [Line Items]
Long-term debt, principal amount		$ 98.5	$ 131.5
Interest rate		1.86%
Bonds | New Taiwan Dollar 2.66% due 2026 to 2029
Debt Instrument [Line Items]
Long-term debt, principal amount		$ 197.0	190.6
Interest rate		2.66%
Bonds | New Taiwan Dollar variable-rate 2.75% due 2026 to 2030
Debt Instrument [Line Items]
Long-term debt, principal amount		$ 98.5	94.8
Interest rate, variable percentage		2.75%
Notes Payable, Other Payables | Other 4.07% due 2030 to 2034
Debt Instrument [Line Items]
Long-term debt, principal amount		$ 10.0	9.4
Interest rate, weighted average		4.07%
Related Party | Chinese Renminbi 5.5% due 2026 to 2030 | Related Party
Debt Instrument [Line Items]
Long-term debt, principal amount		$ 236.5	279.8
Interest rate		5.50%
Related Party | Chinese Renminbi 5.7% due 2033 | Related Party
Debt Instrument [Line Items]
Long-term debt, principal amount		$ 0.0	24.6
Interest rate		5.70%
Finance Lease Obligations | Foreign 11.5% due 2026 to 2052
Debt Instrument [Line Items]
Long-term debt, principal amount		$ 54.2	$ 8.0
Interest rate, weighted average		11.50%