Derivative Financial Instruments (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 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	$ 24,746,000		$ 24,746,000		$ 19,504,000
Fair value of derivative instruments - long-term assets	12,418,000		12,418,000		10,878,000
Total derivative assets	37,164,000		37,164,000		30,382,000
Fair value of derivative instruments - current liabilities	(17,871,000)		(17,871,000)		(27,229,000)
Fair value of derivative liabilities - long-term liabilities	(2,010,000)		(2,010,000)		(32,190,000)
Total derivative liabilities	(19,881,000)		(19,881,000)		(59,419,000)
Financial Statement Impact of Derivative Contracts
Unrealized gain (loss)			46,320,000	145,527,000
Total revenue: derivative gain	19,699,000	136,067,000	19,514,000	87,352,000
Total derivative gain related to purchase product costs	20,432,000	51,579,000	31,136,000	32,779,000
Derivative gain (loss) related to facility expenses	(800,000)	1,146,000	(468,000)	2,892,000
Total gain	39,331,000	188,792,000	50,182,000	123,023,000
Commodity contracts
Derivative financial instruments
Direct product positions as a percentage of derivative positions used to manage future commodity price exposure	73.00%		73.00%
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		9,000
Estimated fair value of embedded derivative contract liability including portion allocable to host processing agreement	61,587,000		61,587,000
Inception value for period from April 1, 2015 to December 31, 2022	(53,507,000)		(53,507,000)
Recorded value of embedded derivative contract liability	8,080,000		8,080,000
Embedded derivative in electricity purchase contract
Derivative financial instruments
Estimated fair value of embedded derivative contract asset	5,700,000		5,700,000
Derivative instruments not designated as hedging instruments
Derivative financial instruments
Notional quantity of crude oil contract (in bbl)	1,885,366		1,885,366
Notional quantity of natural gas contract (in MMBtu)	5,173,965		5,173,965
Natural Gas Liquids (gal)	133,196,994		133,196,994
Derivative contracts not designated as hedging instruments
Fair value of derivative instruments - current assets	24,746,000		24,746,000		19,504,000
Fair value of derivative instruments - long-term assets	12,418,000		12,418,000		10,878,000
Total derivative assets	37,164,000		37,164,000		30,382,000
Fair value of derivative instruments - current liabilities	(17,871,000)		(17,871,000)		(27,229,000)
Fair value of derivative liabilities - long-term liabilities	(2,010,000)		(2,010,000)		(32,190,000)
Total derivative liabilities	(19,881,000)		(19,881,000)		(59,419,000)
Derivative instruments not designated as hedging instruments | Derivative revenue
Financial Statement Impact of Derivative Contracts
Realized gain (loss)	3,089,000	2,841,000	6,987,000	(7,637,000)
Unrealized gain (loss)	16,610,000	133,226,000	12,527,000	94,989,000
Total revenue: derivative gain	19,699,000	136,067,000	19,514,000	87,352,000
Derivative instruments not designated as hedging instruments | Derivative gain (loss) related to purchased product costs
Financial Statement Impact of Derivative Contracts
Realized gain (loss)	(1,045,000)	(7,793,000)	(3,125,000)	(14,867,000)
Unrealized gain (loss)	21,477,000	59,372,000	34,261,000	47,646,000
Total derivative gain related to purchase product costs	20,432,000	51,579,000	31,136,000	32,779,000
Derivative instruments not designated as hedging instruments | Derivative (loss) gain related to facility expenses
Financial Statement Impact of Derivative Contracts
Unrealized gain (loss)	$ (800,000)	$ 1,146,000	$ (468,000)	$ 2,892,000