Derivative Instruments and Hedging Activities - Fair Value of Derivative Assets and Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2022	Sep. 30, 2021	Mar. 31, 2021
Derivative assets:
Total derivative assets — gross	$ 2,035	$ 1,687	$ 260
Gross amounts offset in the balance sheet	(215)	(216)	(35)
Cash collateral received	(603)	(468)	(37)
Total derivative assets — net	1,217	1,003	188
Derivative liabilities:
Total derivative liabilities — gross	(324)	(317)	(123)
Gross amounts offset in the balance sheet	215	216	35
Cash collateral pledged	0	3	0
Total derivative liabilities — net	(109)	(98)	(88)
Commodity contracts
Derivative assets:
Total derivative assets — gross	38	58	2
Derivative liabilities:
Total derivative liabilities — gross	(17)	(23)	(1)
Derivatives designated as hedging instruments:
Derivative assets:
Total derivative assets — gross	49	20	16
Derivatives designated as hedging instruments: | Foreign currency contracts
Derivative assets:
Total derivative assets — gross	18	20	16
Derivatives designated as hedging instruments: | Interest rate contracts
Derivative assets:
Total derivative assets — gross	31	0	0
Derivative liabilities:
Total derivative liabilities — gross	(1)	(29)	(39)
Derivatives not designated as hedging instruments:
Derivative assets:
Total derivative assets — gross	1,948	1,609	242
Derivative liabilities:
Total derivative liabilities — gross	(306)	(265)	(83)
Derivatives not designated as hedging instruments: | Foreign currency contracts
Derivative assets:
Total derivative assets — gross	17	18	9
Derivative liabilities:
Total derivative liabilities — gross	(2)	(8)	(12)
Derivatives not designated as hedging instruments: | Commodity contracts
Derivative assets:
Total derivative assets — gross	1,931	1,591	233
Derivative liabilities:
Total derivative liabilities — gross	$ (304)	$ (257)	$ (71)