Oil and Gas Reserve Information - Unaudited - Proved reserves (Details)	12 Months Ended
	Dec. 31, 2012 MBbls	Dec. 31, 2011 MBbls	Dec. 31, 2010 MBbls
Oil [Member]
Proved Developed and Undeveloped Reserves [Abstract]
Proved reserves beginning balance	1,395,000	1,623,000	1,931,000
Revisions of previous estimates	215,000	(294,000)	187,000
Extensions, discoveries and other additions	647,000	595,000	168,000
Purchase of producing properties		43,000	0
Sale of reserves in place	(81,000)		0
Production	(521,000)	(572,000)	(663,000)
Proved reserves ending balance	1,655,000	1,395,000	1,623,000
Proved developed reserves	1,225,000	1,160,000	1,474,000
Proved undeveloped reserve	430,000	235,000	149,000
Natural Gas Liquids [Member]
Proved Developed and Undeveloped Reserves [Abstract]
Proved reserves, ending balance	15,111,000	8,373,000	10,508,000
Revisions of previous estimates	(958,000)	308,000	187,000
Extensions, discoveries and other additions	14,572,000	8,627,000	150,000
Purchase of producing properties		91,000	0
Sale of reserves in place	0		0
Production	(3,365,000)	(2,288,000)	(2,472,000)
Proved reserves, ending balance	25,360,000	15,111,000	8,373,000
Proved developed reserves	4,752,000	4,040,000	2,295,000
Proved undeveloped reserve	20,608,000	11,071,000	6,078,000
Natural Gas [Member]
Proved Developed and Undeveloped Reserves [Abstract]
Proved reserves beginning balance	241,926,000	174,566,000	156,853,000
Revisions of previous estimates	(52,076,000)	8,418,000	20,958,000
Extensions, discoveries and other additions	46,390,000	82,113,000	47,681,000
Purchase of producing properties		1,292,000	2,336,000
Sale of reserves in place	(15,806,000)		(28,761,000)
Production	(27,466,000)	(24,463,000)	(24,501,000)
Proved reserves ending balance	192,968,000	241,926,000	174,566,000
Proved developed reserves	140,307,000	143,441,000	110,599,000
Proved undeveloped reserve	52,661,000	98,485,000	63,967,000
Oil and Natural Gas Reserves [Member]
Proved Developed and Undeveloped Reserves [Abstract]
Proved reserves, ending balance	265,407,000	192,677,000	178,947,000
Revisions of previous estimates	(51,744,000)	6,962,000	22,267,000
Extensions, discoveries and other additions	64,844,000	94,310,000	48,839,000
Purchase of producing properties		1,641,000	2,336,000
Sale of reserves in place	(16,292,000)		(28,761,000)
Production	(33,957,000)	(30,183,000)	(30,951,000)
Proved reserves, ending balance	228,258,000	265,407,000	192,677,000
Proved developed reserves	59,993,000	103,935,000	67,156,000
Proved undeveloped reserve	168,265,000	161,472,000	125,521,000