Risk Management - FV Input Level - Assets (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Gross amount	$ 460	$ 551
Energy commodity derivative contracts
Derivative [Line Items]
Gross amount	56	221
Contracts available for netting	(19)	(39)
Cash collateral held(b)	(21)	(25)
Net amount	16	157
Interest rate contracts
Derivative [Line Items]
Gross amount	358	133
Contracts available for netting	0	(7)
Cash collateral held(b)	0	0
Net amount	358	126
Foreign currency contracts
Derivative [Line Items]
Gross amount	46	197
Contracts available for netting	(6)	(6)
Cash collateral held(b)	0	0
Net amount	40	191
Level 1 | Energy commodity derivative contracts
Derivative [Line Items]
Gross amount	19	28
Level 1 | Interest rate contracts
Derivative [Line Items]
Gross amount	0	0
Level 1 | Foreign currency contracts
Derivative [Line Items]
Gross amount	0	0
Level 2 | Energy commodity derivative contracts
Derivative [Line Items]
Gross amount	37	193
Level 2 | Interest rate contracts
Derivative [Line Items]
Gross amount	358	133
Level 2 | Foreign currency contracts
Derivative [Line Items]
Gross amount	46	197
Level 3 | Energy commodity derivative contracts
Derivative [Line Items]
Gross amount	0	0
Level 3 | Interest rate contracts
Derivative [Line Items]
Gross amount	0	0
Level 3 | Foreign currency contracts
Derivative [Line Items]
Gross amount	$ 0	$ 0