Suspended Well Costs (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Capitalized Exploratory Well Costs that Have Been Capitalized for Period of One Year or Less		$ 9,100	$ 15,618	$ 35,862
Capitalized Exploratory Well Costs that Have Been Capitalized for Period Greater than One Year		0	2,982	0
Capitalized Exploratory Well Costs		9,100	18,600	35,862
Increase (Decrease) in Capitalized Exploratory Well Costs that are Pending Determination of Proved Reserves [Roll Forward]
Capitalized Exploratory Well Costs Beginning Balance		18,600	35,862	34,384
Capitalized Exploratory Well Cost, Additions Pending Determination of Proved Reserves		9,100	15,618	35,862
Reclassification to Well, Facilities, and Equipment Based on Determination of Proved Reserves		(5,865)	(32,880)	(34,384)
Capitalized Exploratory Well Cost, Charged to Expense	3,600	(12,735)	0	0
Capitalized Exploratory Well Costs Ending Balance		$ 9,100	$ 18,600	$ 35,862
Projects that have Exploratory Well Costs that have been Capitalized for Period Greater than One Year, Number of Projects		0	2	0