Supplemental Oil and Gas Reserve Information (Details 3)	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Oil
Average Sales Price and Production Costs Per Unit of Production [Line Items]
Average sales price	83.09	[1],[2]	87.61	[1],[2]	72.36	[1],[2]
Gas
Average Sales Price and Production Costs Per Unit of Production [Line Items]
Average sales price	3.70	[1]	5.31	[1]	5.44	[1]

[1] Average prices for December 31, 2012, 2011 and 2010 were based on the 12-month unweighted arithmetic average of the first-day-of-the-month price for the period from January through December during each respective calendar year.

[2] Includes natural gas liquids.