Fair Value of FInancial Instruments Fair Value Unobservable Input Reconciliation (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Net realized and unrealized losses	$ 35,000,000	$ 11,000,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	(1,000,000)
Net transfers out of Level 3 to Level 2	(15,000,000)	(10,000,000)
Net liabilities from price risk management activities as of end of year	139,000,000	119,000,000	$ 119,000,000
Level 3 net unrealized losses that have been fully offset by the effect of regulatory accounting	41,000,000	11,000,000
Net realized losses	$ 6,000,000	$ 0