Fair Value of FInancial Instruments Fair Value Unobservable Input Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Net realized and unrealized losses	$ (40)	$ 35
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	0
Net transfers out of Level 3 to Level 2	(11)	(15)
Net liabilities from price risk management activities as of end of year	88	139	$ 119
Level 3 net unrealized losses that have been fully offset by the effect of regulatory accounting	32	41
Net realized losses	$ 8	$ 6