Derivative Instruments and Hedging Activities - Fair Value of Derivative Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Derivative assets:
Total derivative assets — gross	$ 185	$ 139	$ 89
Gross amounts offset in the balance sheet	(48)	(57)	(27)
Cash collateral received	(6)	0	(2)
Total derivative assets — net	131	82	60
Derivative liabilities:
Total derivative liabilities — gross	(147)	(187)	(198)
Gross amounts offset in the balance sheet	48	57	27
Cash collateral pledged	0	7	11
Total derivative liabilities — net	(99)	(123)	(160)
Derivatives subject to PGC and DS mechanisms:
Derivative assets:
Total derivative assets — gross	1	7	0
Derivative liabilities:
Total derivative liabilities — gross	(2)	0	(3)
Derivatives designated as hedging instruments:
Derivative assets:
Total derivative assets — gross	8	17	17
Derivatives designated as hedging instruments: | Foreign currency contracts
Derivative assets:
Total derivative assets — gross	8	17	14
Derivatives designated as hedging instruments: | Interest rate contracts
Derivative assets:
Total derivative assets — gross	0	0	3
Derivative liabilities:
Total derivative liabilities — gross	(50)	(55)	(7)
Derivatives not designated as hedging instruments:
Derivative assets:
Total derivative assets — gross	176	115	72
Derivative liabilities:
Total derivative liabilities — gross	(95)	(132)	(188)
Derivatives not designated as hedging instruments: | Foreign currency contracts
Derivative assets:
Total derivative assets — gross	6	15	26
Derivative liabilities:
Total derivative liabilities — gross	(25)	(14)	(4)
Derivatives not designated as hedging instruments: | Commodity contracts
Derivative assets:
Total derivative assets — gross	170	100	46
Derivative liabilities:
Total derivative liabilities — gross	$ (70)	$ (118)	$ (184)