Derivative Instrument and Hedging Activities 3 (Details) (USD $) In Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Cash Flow Hedges [Abstract]
Change in fair value - increase in Accumulated other comprehensive loss, excluding tax expense effect of $6,891, $1,308, $7,757 and $3,614, respectively	$ 18,478	$ 3,253	$ 20,748	$ 8,989
Reclassification of unrealized loss from Accumulated other comprehensive loss - increase of Interest expense, excluding tax expense effect of $2,253, $2,250, $4,481 and $4,554, respectively	5,615	5,607	11,166	11,357
Commodity Contracts Gathering and Processing [Abstract]
Change in fair value - increase/(decrease) in Accumulated other comprehensive loss, excluding tax expense effect of $(194), $390, $365 and $(9,613), respectively	(538)	1,080	1,014	(26,674)
Reclassification of unreazlied gain from Accumulated other comprehensive loss - increase of Operating revenues, exluding tax expense effect of $1,678, $1,830, $3,310 and $2,272, respectively	4,655	5,077	9,184	6,304
Economic Hedges Distribution [Abstract]
Change in fair value - increase/(decrease) in Deferred natural gas purchases	(4,279)	(23,947)	(23,772)	(7,707)
Economic Hedges Gathering and Processing [Abstract]
Change in fair value of strategic hedges - (increase)/decrease in Opearating revenues	7,149	(21,597)	23,865	(14,672)
Change in fair value of other hedges - (increase)/decrease in Operating revenues	$ (18)	$ (375)	$ (217)	$ 186