Reclassification From Accumulated Other Comprehensive Loss (Amounts Reclassified From Accumulated Other Comprehensive Loss) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013
Gas sales	$ 793,352		$ 504,496
General and administrative expenses	56,387		37,215
Income before income taxes	323,185		212,612
Provision for income taxes	128,995	[1]	85,097	[1]
Net Income	194,190		127,515
Reclassification Out Of Accumulated Other Comprehensive Loss [Member]
Net income	(26,431)
Gains (Losses) On Cash Flow Hedges [Member] | Reclassification Out Of Accumulated Other Comprehensive Loss [Member]
Income before income taxes	(43,918)
Provision for income taxes	(17,568)
Net income	(26,350)
Gains (Losses) On Cash Flow Hedges [Member] | Reclassification Out Of Accumulated Other Comprehensive Loss [Member] | Settlements [Member]
Gas sales	(41,978)
Gains (Losses) On Cash Flow Hedges [Member] | Reclassification Out Of Accumulated Other Comprehensive Loss [Member] | Ineffectiveness [Member]
Gas sales	(1,940)
Pension And Other Postretirement [Member] | Reclassification Out Of Accumulated Other Comprehensive Loss [Member]
General and administrative expenses	(135)	[2]
Income before income taxes	(135)
Provision for income taxes	(54)
Net Income	$ (81)

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

[2] Included in the computation of net periodic pension cost (see Footnote 13 for additional details).