Fair Value Measurements - Fair Value Measurements (Details) - USD ($) $ in Millions	Sep. 30, 2025	Sep. 30, 2024
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 12	$ 64
Investments
Government debt securities	473	457
Corporate debt securities	411	378
Mortgage and asset-backed securities	40	43
Institutional mutual funds	367	342
Forward debt securities contracts - asset	16	11
Cash, Cash Equivalents, and Short-term Investments	274	278
Private equity funds measured at net asset value	875	738
Private real estate measured at net asset value	467	432
Private credit measured at net asset value	278	219
Commingled funds measured at net asset value	1,528	1,300
Total investments	5,573	4,968
Commodity contract derivatives	16	7
Total	5,597	4,975
Liabilities
Currency swaps	115	116	[1]
Interest rate swaps	698	840
Commodity contract derivatives	6	5
Total	884	1,122
Equity Securities, FV-NI, Noncurrent	844	770
Derivative Liability, Subject to Master Netting Arrangement, after Offset	884	1,122
Derivative Asset, Subject to Master Netting Arrangement, after Offset	24	7
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Cash, Cash Equivalents, and Short-term Investments	274	278
Net payables	101	47
Payable, Investment, Purchase	60	78
Government debt securities reclassed	111	95
Commingled funds reclassed		495
Repurchase Agreements	124	174
Commodity Contract under FHP
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	(57)	(161)
Liabilities
Derivative Liability, Subject to Master Netting Arrangement, after Offset	65	161
Derivative Asset, Subject to Master Netting Arrangement, after Offset	8	0
Fair Value, Inputs, Level 1
Investments
Government debt securities	423	400
Corporate debt securities	0	0
Mortgage and asset-backed securities	0	0
Institutional mutual funds	367	342
Forward debt securities contracts - asset	0	0
Cash, Cash Equivalents, and Short-term Investments	111	95
Private equity funds measured at net asset value	0	0
Private real estate measured at net asset value	0	0
Private credit measured at net asset value	0	0
Commingled funds measured at net asset value	0	0
Total investments	1,745	1,607
Commodity contract derivatives	0	0
Total	1,745	1,607
Liabilities
Currency swaps	0	0
Interest rate swaps	0	0
Commodity contract derivatives	0	0
Total	0	0
Equity Securities, FV-NI, Noncurrent	844	770
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Cash, Cash Equivalents, and Short-term Investments	111	95
Fair Value, Inputs, Level 1 | Commodity Contract under FHP
Liabilities
Derivative Liability, Subject to Master Netting Arrangement, after Offset	0	0
Derivative Asset, Subject to Master Netting Arrangement, after Offset	0
Fair Value, Inputs, Level 2
Investments
Government debt securities	50	57
Corporate debt securities	411	378
Mortgage and asset-backed securities	40	43
Institutional mutual funds	0	0
Forward debt securities contracts - asset	16	11
Cash, Cash Equivalents, and Short-term Investments	163	183
Private equity funds measured at net asset value	0	0
Private real estate measured at net asset value	0	0
Private credit measured at net asset value	0	0
Commingled funds measured at net asset value	0	0
Total investments	680	672
Commodity contract derivatives	16	7
Total	704	679
Liabilities
Currency swaps	115	116
Interest rate swaps	698	840
Commodity contract derivatives	6	5
Total	884	1,122
Equity Securities, FV-NI, Noncurrent	0	0
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Cash, Cash Equivalents, and Short-term Investments	163	183
Fair Value, Inputs, Level 2 | Commodity Contract under FHP
Liabilities
Derivative Liability, Subject to Master Netting Arrangement, after Offset	65	161
Derivative Asset, Subject to Master Netting Arrangement, after Offset	8
Fair Value, Inputs, Level 3
Investments
Government debt securities	0	0
Corporate debt securities	0	0
Mortgage and asset-backed securities	0	0
Institutional mutual funds	0	0
Forward debt securities contracts - asset	0	0
Cash, Cash Equivalents, and Short-term Investments	0	0
Private equity funds measured at net asset value	0	0
Private real estate measured at net asset value	0	0
Private credit measured at net asset value	0	0
Commingled funds measured at net asset value	0	0
Total investments	0	0
Commodity contract derivatives	0	0
Total	0	0
Liabilities
Currency swaps	0	0
Interest rate swaps	0	0
Commodity contract derivatives	0	0
Total	0	0
Equity Securities, FV-NI, Noncurrent	0	0
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Cash, Cash Equivalents, and Short-term Investments	0	0
Fair Value, Inputs, Level 3 | Commodity Contract under FHP
Liabilities
Derivative Liability, Subject to Master Netting Arrangement, after Offset	0	$ 0
Derivative Asset, Subject to Master Netting Arrangement, after Offset	$ 0

[1] See Note 16 — Risk Management Activities and Derivative Transactions — Offsetting of Derivative Assets and Liabilities.