Reclassifications From Accumulated Other Comprehensive Income (Amounts Reclassified From Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gas sales		$ 1,946	$ 2,827	$ 2,381
Gain (loss) on derivatives		47	139	26
Net actuarial loss	[1]	(3)	(15)	11
General and administrative expenses		246	221	191
Provision (benefit) for income taxes		(2,005)	525	486
Net Income (Loss)		(4,556)	$ 924	$ 704
Reclassification from/to Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net Income (Loss)		(128)
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		(81)
Net Income (Loss)		(127)
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		(209)
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]
Net actuarial loss		(3)
General and administrative expenses		2
Net Income (Loss)		$ (1)

[1] Net of $0, ($10), and $8 million in taxes for the years ended December 31, 2015, 2014 and 2013, respectively.