Supplemental Crude Oil and Natural Gas Information - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2012 M	Dec. 31, 2011 M	Dec. 31, 2010 M
Reserve Quantities [Line Items]
Percentage of discounted future net cash flows	99.00%	96.00%	94.00%
Percent of proved crude oil reserve estimates prepared by external reserve engineers	99.00%
Percent of proved natural gas reserve estimates prepared by external reserve engineers	96.00%
Commodity prices utilized for reserve estimates (in months)	12	12	12
Discount factor utilized as standardized measure for future net cash flows	10.00%
Crude Oil [Member]
Reserve Quantities [Line Items]
Weighted average price utilized in computation of future cash inflows	86.56	88.71	71.92
Natural Gas [Member]
Reserve Quantities [Line Items]
Weighted average price utilized in computation of future cash inflows	4.31	5.59	5.07