Reclassifications From Accumulated Other Comprehensive Income (Amounts Reclassified From AOCI Into Earnings) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Gas sales	$ 504,496		$ 462,134
General and administrative expenses	37,215		48,826
Income before income taxes	212,612		178,422
Provision for income taxes	85,097	[1]	70,718	[1]
Net income	127,515		107,704
Parent [Member]
Income before income taxes	127,515		107,704
Net income	127,515		107,704
Reclassification out of Accumulated Other Comprehensive Income [Member]
Net income	(47,687)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Gains and Losses on Cash Flow Hedges [Member]
Income before income taxes	(79,923)
Provision for income taxes	(31,969)
Net income	(47,954)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Pension and Other Postretirement [Member]
General and administrative expenses	445	[2]
Income before income taxes	445
Provision for income taxes	178
Net income	267
Reclassification out of Accumulated Other Comprehensive Income [Member] | Settlements [Member] | Gains and Losses on Cash Flow Hedges [Member]
Gas sales	(78,621)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Ineffectiveness [Member] | Gains and Losses on Cash Flow Hedges [Member]
Gas sales	$ (1,302)

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

[2] This accumulated other comprehensive income component is included in the computation of net periodic pension cost (see pension footnote for additional details.)