Reclassification From Accumulated Other Comprehensive Loss (Amounts Reclassified From Accumulated Other Comprehensive Loss) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gas sales	$ 717,000		$ 614,000		$ 1,510,000		$ 1,119,000
Income before income taxes	347,000		410,000		670,000		622,000
Provision for income taxes	140,000	[1]	164,000	[1]	269,000	[1]	249,000	[1]
Net Income	207,000		246,000		401,000		373,000
Reclassification Out Of Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net Income					(41)
Gains (Losses) On Cash Flow Hedges [Member] | Reclassification Out Of Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes					(69)
Provision for income taxes					(28)
Net Income					(41)
Gains (Losses) On Cash Flow Hedges [Member] | Reclassification Out Of Accumulated Other Comprehensive Loss [Member] | Settlements [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gas sales					(67)
Gains (Losses) On Cash Flow Hedges [Member] | Reclassification Out Of Accumulated Other Comprehensive Loss [Member] | Ineffectiveness [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gas sales					$ (2)

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