Derivatives and Risk Management Activities - Broker Receivable/Payable (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Offsetting Assets, Liabilities [Line Items]
Initial margin	$ 139	$ 48
Variation margin posted	222	164
Letters of credit	(181)	(166)
Net broker receivable	259	$ 212
Exchange traded
Offsetting Assets, Liabilities [Line Items]
Letters of credit	$ (102)