Derivative Instruments and Hedging Activities - Fair Value of Derivative Assets and Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2021	Sep. 30, 2020	Mar. 31, 2020
Derivative assets:
Total derivative assets — gross	$ 260	$ 139	$ 107
Gross amounts offset in the balance sheet	(35)	(57)	(26)
Cash collateral received	(37)	0	0
Total derivative assets — net	188	82	81
Derivative liabilities:
Total derivative liabilities — gross	(123)	(187)	(375)
Gross amounts offset in the balance sheet	35	57	26
Cash collateral pledged	0	7	58
Total derivative liabilities — net	(88)	(123)	(291)
Commodity contracts
Derivative assets:
Total derivative assets — gross	2	7	1
Derivative liabilities:
Total derivative liabilities — gross	(1)	0	(2)
Derivatives designated as hedging instruments: | Foreign currency contracts
Derivative assets:
Total derivative assets — gross	16	17	28
Derivatives designated as hedging instruments: | Interest rate contracts
Derivative liabilities:
Total derivative liabilities — gross	(39)	(55)	(54)
Derivatives not designated as hedging instruments:
Derivative assets:
Total derivative assets — gross	242	115	78
Derivative liabilities:
Total derivative liabilities — gross	(83)	(132)	(319)
Derivatives not designated as hedging instruments: | Foreign currency contracts
Derivative assets:
Total derivative assets — gross	9	15	31
Derivative liabilities:
Total derivative liabilities — gross	(12)	(14)	(7)
Derivatives not designated as hedging instruments: | Commodity contracts
Derivative assets:
Total derivative assets — gross	233	100	47
Derivative liabilities:
Total derivative liabilities — gross	$ (71)	$ (118)	$ (312)