Derivative Instruments and Hedging Activities - Schedule of Derivative Assets, Liabilities and the Effects of Offsetting (Details) - USD ($) $ in Millions	Sep. 30, 2021	Sep. 30, 2020
Derivative assets:
Fair value of derivative instruments	$ 1,687	$ 139
Gross amounts offset in the balance sheet	(216)	(57)
Cash collateral received	(468)	0
Total derivative assets – net	1,003	82
Derivative liabilities:
Total derivative liabilities – gross	(317)	(187)
Gross amounts offset in the balance sheet	216	57
Cash collateral pledged	3	7
Total derivative liabilities – net	(98)	(123)
Commodity contract - subject to PGC and DS mechanisms
Derivative assets:
Fair value of derivative instruments	58	7
Derivative liabilities:
Total derivative liabilities – gross	(23)	0
Derivatives designated as hedging instruments | Foreign currency contracts
Derivative assets:
Fair value of derivative instruments	20	17
Derivatives designated as hedging instruments | Interest rate contracts
Derivative liabilities:
Total derivative liabilities – gross	(29)	(55)
Derivatives not designated as hedging instruments
Derivative assets:
Fair value of derivative instruments	1,609	115
Derivative liabilities:
Total derivative liabilities – gross	(265)	(132)
Derivatives not designated as hedging instruments | Foreign currency contracts
Derivative assets:
Fair value of derivative instruments	18	15
Derivative liabilities:
Total derivative liabilities – gross	(8)	(14)
Derivatives not designated as hedging instruments | Commodity contracts
Derivative assets:
Fair value of derivative instruments	1,591	100
Derivative liabilities:
Total derivative liabilities – gross	$ (257)	$ (118)