Reclassification From Accumulated Other Comprehensive Income (Amounts Reclassified From Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gas sales	$ 458	$ 645	$ 1,540	$ 2,155
Gain (loss) on derivatives	15	78	30	(29)
General and administrative expenses	60	54	188	162
Provision (benefit) for income taxes	(1,088)	140	(1,532)	409
Net Income (Loss)	$ (1,739)	$ 211	(2,449)	$ 612
Reclassification from/to Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net Income (Loss)			(87)
Cash Flow Hedges [Member] | Reclassification from/to Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Provision (benefit) for income taxes			(56)
Net Income (Loss)			(88)
Cash Flow Hedges [Member] | Reclassification from/to Accumulated Other Comprehensive Income [Member] | Settlements [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gas sales			(145)
Cash Flow Hedges [Member] | Reclassification from/to Accumulated Other Comprehensive Income [Member] | Ineffectiveness [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gain (loss) on derivatives			1
Pension And Other Postretirement [Member] | Reclassification from/to Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
General and administrative expenses			2
Provision (benefit) for income taxes			1
Net Income (Loss)			$ 1