Derivatives And Fair Value Measurements (Carrying Value Of Derivatives) (Details) (USD $) In Thousands	1 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended		9 Months Ended		3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended
	Aug. 31, 2011	Sep. 30, 2010	Sep. 30, 2010	Sep. 30, 2011 Commodity Derivatives [Member]	Sep. 30, 2010 Commodity Derivatives [Member]	Sep. 30, 2011 Commodity Derivatives [Member]	Sep. 30, 2010 Commodity Derivatives [Member]	Sep. 30, 2010 Gas Purchase Commitment [Member]	Sep. 30, 2010 Gas Purchase Commitment [Member]	Sep. 30, 2010 Fair Value Derivatives [Member]	Sep. 30, 2010 Fair Value Derivatives [Member]
Derivatives, Fair Value [Line Items]
Derivative fair value at beginning of period		$ 200,473	$ 105,364	$ 116,349	$ 193,394	$ 146,762	$ 107,881	$ (6,161)	$ (6,625)	$ 13,240	$ 4,108
Change in net amounts receivable and payable		(4,626)	(10,276)	(576)	(234)	(960)	(1,096)			(4,392)	(9,180)
Net settlements reported in revenue		(54,716)	(136,349)	(16,815)	(54,716)	(56,143)	(136,349)
Net settlements reported in interest expense			(10,848)								(10,848)
Cash settlements reported in long-term debt		(12,134)	(30,816)							(12,134)	(30,816)
Unrealized change in fair value of Gas Purchase Commitment reported in costs of purchased gas		5,496	5,960					5,496	5,960
Change in fair value of effective interest swaps		3,286	46,736							3,286	46,736
Ineffectiveness reported in other revenue		(812)	(2,399)	880	(812)	1,698	(2,399)
Unrealized gains reported in other revenue	48,900			29,737		48,852
Unrealized gains (losses) reported in OCI		89,627	259,222	76,598	89,627	65,964	259,222
Derivative fair value at end of period		$ 226,594	$ 226,594	$ 206,173	$ 227,259	$ 206,173	$ 227,259	$ (665)	$ (665)