Fair Value of FInancial Instruments Fair Value Unobservable Input Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Net realized and unrealized losses	$ 80	$ 15
Settlements	0	(4)
Net transfers out of Level 3 to Level 2	(61)	(50)
Net liabilities from price risk management activities as of end of year	119	100	$ 139
Level 3 net unrealized losses that have been fully offset by the effect of regulatory accounting	$ 80	12
Net realized losses		$ 3