Fair Value of FInancial Instruments Fair Value Unobservable Input Reconciliation (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Net realized and unrealized losses	$ 11,000,000	$ 80,000,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	(1,000,000)	0
Net transfers out of Level 3 to Level 2	(10,000,000)	(61,000,000)
Net liabilities from price risk management activities as of end of year	119,000,000	119,000,000	$ 100,000,000
Level 3 net unrealized losses that have been fully offset by the effect of regulatory accounting	11,000,000	80,000,000
Net realized losses	$ 0	$ 0