Oil and Gas Reserves Information (Unaudited) - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Average Sales Price and Production Costs Per Unit of Production [Line Items]
Annual Change in Total Proved Undeveloped Natural Gas Reserves	(460,000)
Crude Oil and NGL Per Barrel [Member]
Average Sales Price and Production Costs Per Unit of Production [Line Items]
Average sales prices	94.61	92.93
Natural Gas Per Thousand Cubic Feet [Member]
Average Sales Price and Production Costs Per Unit of Production [Line Items]
Average sales prices	2.84	4.18