Derivatives and Risk Management Activities - Broker Receivable/Payable (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Offsetting Assets, Liabilities [Line Items]
Initial margin	$ 103	$ 73
Variation margin posted/(returned)	105	(45)
Letters of credit	(140)	(157)
Net broker receivable	133
Net broker payable		(45)
Exchange traded
Offsetting Assets, Liabilities [Line Items]
Letters of credit	$ (75)	$ (73)