Risk Management - FV Input Level - Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Gross amount	$ (65)	$ (171)
Energy commodity derivative contracts
Derivative [Line Items]
Gross amount	(58)	(50)
Contracts available for netting	19	39
Collateral posted(b)	0	0
Net amount	(39)	(11)
Interest rate contracts
Derivative [Line Items]
Gross amount	(1)	(115)
Contracts available for netting	0	7
Collateral posted(b)	0	0
Net amount	(1)	(108)
Foreign currency contracts
Derivative [Line Items]
Gross amount	(6)	(6)
Contracts available for netting	6	6
Collateral posted(b)	0	0
Net amount	0	0
Level 1 | Energy commodity derivative contracts
Derivative [Line Items]
Gross amount	(3)	(11)
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	(55)	(39)
Level 2 | Interest rate contracts
Derivative [Line Items]
Gross amount	(1)	(115)
Level 2 | Foreign currency contracts
Derivative [Line Items]
Gross amount	(6)	(6)
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