Derivative Instrument and Hedging Activities 3 (Details) (USD $) In Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Cash Flow Hedges [Abstract]
Change in fair value - increase in Accumulated other comprehensive loss, excluding tax expense effect of $18,697, $1,459, $26,454 and $5,073, respectively	$ 50,283	$ 3,631	$ 71,031	$ 12,620
Reclassification of unrealized loss from Accumulated other comprehensive loss - increase of Interest expense, excluding tax expense effect of $2,294, $2,186, $6,775 and $6,740, respectively	5,715	5,447	16,881	16,804
Commodity Contracts Gathering and Processing [Abstract]
Change in fair value - decrease in Accumulated other comprehensive loss, excluding tax expense effect of $3,039, $4,707, $2,674 and $14,320, respectively	8,434	13,059	7,420	39,733
Reclassification of unrealized gain from Accumulated other comprehensive loss - increase of Operating revenues, exluding tax expense effect of $2,017, $2,019, $5,327 and $4,291, respectively	5,598	5,603	14,782	11,907
Economic Hedges Gathering and Processing [Abstract]
Change in fair value of strategic hedges - (increase)/decrease in Operating revenues	3,998	29,180	27,863	14,508
Change in fair value of other hedges - (increase)/decrease in Operating revenues	(35)	279	(252)	465
Economic Hedges Distribution [Abstract]
Change in fair value - (increase) decrease in Deferred natural gas purchases	$ 10,114	$ (13,914)	$ (13,658)	$ (6,207)