Reclassifications From Accumulated Other Comprehensive Income (Amounts Reclassified From AOCI Into Earnings) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Gas sales	$ 617,427		$ 497,219		$ 1,736,101		$ 1,394,745
General and administrative expenses	50,969		36,121		135,754		129,879
Income (loss) before income taxes	310,016		(82,206)		932,470		(561,962)
Provision (benefit) for income taxes	124,149	[1]	(28,153)	[1]	373,457	[1]	(210,481)	[1]
Net income (loss)	185,867		(54,053)		559,013		(351,481)
Parent [Member]
Income (loss) before income taxes	185,867		(54,053)		559,013		(351,481)
Net income (loss)	185,867		(54,053)		559,013		(351,481)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Net income (loss)					128,832
Reclassification out of Accumulated Other Comprehensive Income [Member] | Gains and Losses on Cash Flow Hedges [Member]
Income (loss) before income taxes					216,053
Provision (benefit) for income taxes					86,421
Net income (loss)					129,632
Reclassification out of Accumulated Other Comprehensive Income [Member] | Pension and Other Postretirement [Member]
General and administrative expenses					(1,334)
Income (loss) before income taxes					(1,334)
Provision (benefit) for income taxes					(534)
Net income (loss)					(800)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Settlements [Member] | Gains and Losses on Cash Flow Hedges [Member]
Gas sales					218,236
Reclassification out of Accumulated Other Comprehensive Income [Member] | Ineffectiveness [Member] | Gains and Losses on Cash Flow Hedges [Member]
Gas sales					$ (2,183)

[1] Interest expense and the provision for income taxes by segment are allocated as they are incurred at the corporate level.