Derivative Instrument and Hedging Activities 4 Parentheticals (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Cash Flow Hedges Tax [Abstract]
Change in fair value of interest rate hedges tax	$ 18,697,000	$ 1,459,000	$ 26,454,000	$ 5,073,000
Reclassification of unrealized loss (gain) on interest rate hedges tax	2,294,000	2,186,000	6,775,000	6,740,000
Commodity Contracts Gathering and Processing Tax [Abstract]
Change in fair value of commodity hedges tax	3,039,000	4,707,000	2,674,000	14,320,000
Relcassification of unrealized gain (loss) on commodity hedges into earnings tax	2,017,000	2,019,000	5,327,000	4,291,000
Strategic Hedges [Abstract]
Derivative Cash Received On Hedge	6,300,000	7,700,000	23,100,000	27,700,000
Unrealized (gain) loss on commodity derivatives	$ (5,100,000)	$ 29,200,000	$ 9,283,000	$ 12,589,000