Derivative Financial Instruments (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Derivative financial instruments
Producer's option to extend successive years, number	5 years
Derivative contracts not designated as hedging instruments
Fair value of derivative instruments - current assets	$ 16,814,000		$ 19,504,000
Fair value of derivative instruments - long-term assets	10,181,000		10,878,000
Total derivative assets	26,995,000		30,382,000
Fair value of derivative instruments - current liabilities	(23,060,000)		(27,229,000)
Fair value of derivative liabilities - long-term liabilities	(23,938,000)		(32,190,000)
Total derivative liabilities	(46,998,000)		(59,419,000)
Financial Statement Impact of Derivative Contracts
Unrealized gain (loss)	9,033,000	(48,217,000)
Total revenue: derivative loss	(185,000)	(48,715,000)
Total derivative gain (loss) related to purchase product costs	10,704,000	(18,800,000)
Derivative gain related to facility expenses	332,000	1,746,000
Total gain (loss)	10,851,000	(65,769,000)
Commodity contracts
Derivative financial instruments
Direct product hedges as a percentage of derivative positions used to manage future commodity price exposure	70.00%
Embedded derivative in natural gas processing and purchase contract
Derivative financial instruments
Notional amount for embedded derivative in commodity contract (in Dth per day)	9,000
Estimated fair value of embedded derivative contract liability including portion allocable to host processing agreement	84,866,000
Inception value for period from April 1, 2015 to December 31, 2022	(53,507,000)
Recorded value of embedded derivative contract liability	31,359,000
Embedded derivative in electricity purchase contract
Derivative financial instruments
Estimated fair value of embedded derivative contract asset	6,500,000
Derivative instruments not designated as hedging instruments
Derivative financial instruments
Notional quantity of crude oil contract (in bbl)	2,626,742
Notional quantity of natural gas contract (in MMBtu)	4,570,733
Natural Gas Liquids (gal)	101,249,566
Derivative contracts not designated as hedging instruments
Fair value of derivative instruments - current assets	16,814,000		19,504,000
Fair value of derivative instruments - long-term assets	10,181,000		10,878,000
Total derivative assets	26,995,000		30,382,000
Fair value of derivative instruments - current liabilities	(23,060,000)		(27,229,000)
Fair value of derivative liabilities - long-term liabilities	(23,938,000)		(32,190,000)
Total derivative liabilities	(46,998,000)		(59,419,000)
Derivative instruments not designated as hedging instruments | Derivative revenue
Financial Statement Impact of Derivative Contracts
Realized gain (loss)	3,898,000	(10,478,000)
Unrealized gain (loss)	(4,083,000)	(38,237,000)
Total revenue: derivative loss	(185,000)	(48,715,000)
Derivative instruments not designated as hedging instruments | Derivative (loss) gain related to purchased product costs
Financial Statement Impact of Derivative Contracts
Realized gain (loss)	(2,080,000)	(7,074,000)
Unrealized gain (loss)	12,784,000	(11,726,000)
Total derivative gain (loss) related to purchase product costs	10,704,000	(18,800,000)
Derivative instruments not designated as hedging instruments | Derivative (loss) gain related to facility expenses
Financial Statement Impact of Derivative Contracts
Unrealized gain (loss)	$ 332,000	$ 1,746,000