Supplemental Information on Oil & Natural Gas Exploration and Production Activities (unaudited) (Details 2)	12 Months Ended
	Dec. 31, 2012 Boe	Dec. 31, 2011 Boe	Dec. 31, 2010 Boe
Changes in estimated net proved developed crude oil and natural gas reserves
Reserves at the beginning of the period	21,757	86,230	73,046
Revisions of previous estimates	(11,146)	(1,071)	43,572
Production	(6,524)	(18,522)	(30,388)
Reserves sold		(44,880)
Reserves at the end of the period	4,087	21,757	86,230
Oil
Changes in estimated net proved developed crude oil and natural gas reserves
Reserves at the beginning of the period	6,640	47,700	41,710
Revisions of previous estimates	(2,769)	1,469	20,258
Production	(2,951)	(10,056)	(14,268)
Reserves sold		(32,473)
Reserves at the end of the period	920	6,640	47,700
Average prices used in calculations	94.68	95.84	79.79
Gas
Changes in estimated net proved developed crude oil and natural gas reserves
Reserves at the beginning of the period	90,700	231,180	188,020
Revisions of previous estimates	(50,258)	(15,246)	139,882
Production	(21,442)	(50,796)	(96,722)
Reserves sold		(74,438)
Reserves at the end of the period	19,000	90,700	231,180
Average prices used in calculations	2.76	4.15	4.39