Derivative Instruments and Hedging Activities - Schedule of Derivative Assets, Liabilities and the Effects of Offsetting (Details) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Derivative assets:
Total derivative assets – gross	$ 139	$ 101
Gross amounts offset in the balance sheet	(57)	(29)
Total derivative assets – net	82	72
Derivative liabilities:
Total derivative liabilities – gross	(187)	(191)
Gross amounts offset in the balance sheet	57	29
Cash collateral pledged	7	29
Total derivative liabilities – net	(123)	(133)
Commodity contract - subject to PGC and DS mechanisms
Derivative assets:
Total derivative assets – gross	7	1
Derivative liabilities:
Total derivative liabilities – gross	0	(4)
Derivatives designated as hedging instruments | Foreign currency contracts
Derivative assets:
Total derivative assets – gross	17	17
Derivatives designated as hedging instruments | Interest rate contracts
Derivative liabilities:
Total derivative liabilities – gross	(55)	(12)
Derivatives not designated as hedging instruments
Derivative assets:
Total derivative assets – gross	115	83
Derivative liabilities:
Total derivative liabilities – gross	(132)	(175)
Derivatives not designated as hedging instruments | Foreign currency contracts
Derivative assets:
Total derivative assets – gross	15	42
Derivative liabilities:
Total derivative liabilities – gross	(14)	(4)
Derivatives not designated as hedging instruments | Commodity contracts
Derivative assets:
Total derivative assets – gross	100	41
Derivative liabilities:
Total derivative liabilities – gross	$ (118)	$ (171)