Risk Management Activities and Derivative Transactions Offsetting of Derivative Assets (Details) - USD ($) $ in Millions	Jun. 30, 2022	Sep. 30, 2021
Offsetting Assets [Line Items]
Gross amounts of recognized liabilities	$ 1,226	$ 1,725
Total derivatives not subject to master netting or similar arrangement	190	250
Forward Contract Derivative Asset, at Fair Value	2	2
Derivative Liability, Not Subject to Master Netting Arrangement	7	3
Derivative Asset, Fair Value, Gross Liability	254	250
Offsetting Derivative Assets [Abstract]
Derivative, Fair Value, Amount Offset Against Collateral, Net	22
Letter of Credit
Offsetting Assets [Line Items]
Amount of letters of credit outstanding for credit facilities	743	1,200
Currency Swap
Offsetting Assets [Line Items]
Gross amounts of recognized liabilities	133	83
Interest Rate Contract
Offsetting Assets [Line Items]
Gross amounts of recognized liabilities	1,074	1,639
Commodity Contract under FHP
Offsetting Assets [Line Items]
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement	34
Total derivatives not subject to master netting or similar arrangement	64	0
Derivative Liability, Not Subject to Master Netting Arrangement	12	0
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement	86
Offsetting Derivative Assets [Abstract]
Derivative Liability, Fair Value of Collateral	10
Fair Value, Inputs, Level 2
Offsetting Assets [Line Items]
Forward Contract Derivative Asset, at Fair Value	2	$ 2
Fair Value, Inputs, Level 2 | Commodity Contract under FHP
Offsetting Assets [Line Items]
Total derivatives not subject to master netting or similar arrangement	64
Derivative Liability, Not Subject to Master Netting Arrangement	$ 12