Derivative Instrument and Hedging Activities 3 (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Cash Flow Hedges [Abstract]
Change in fair value - increase in Accumulated other comprehensive loss, excluding tax expense effect of $27,589, $5,237 and $3,051, respectively	$ 74,059	$ 13,028	$ 7,589
Reclassification of unrealized loss from Accumulated other comprehensive loss - increase of Interest expense, excluding tax expense effect of $(9,012), $(9,019) and $(8,222), respectively	(22,455)	(22,483)	(20,572)
Commodity Contracts Gathering and Processing [Abstract]
Change in fair value - decrease in Accumulated other comprehensive loss, excluding tax expense effect of $(2,598), $(14,093) and $(3,773), respectively	(7,208)	(39,105)	(10,469)
Reclassification of unrealized gain from Accumulated other comprehensive loss - increase of Operating revenues, exluding tax expense effect of $8,536, $6,787 and $16,231, respectively	23,685	18,833	45,035
Economic Hedges Gathering and Processing [Abstract]
Change in fair value of strategic hedges - (increase)/decrease in Operating revenues	29,855	31,154	88,799
Change in fair value of other hedges - (increase)/decrease in Operating revenues	(96)	283	(12)
Economic Hedges Distribution [Abstract]
Change in fair value - (increase) decrease in Deferred natural gas purchases	$ 2,673	$ (6,166)	$ (49,083)