Supplementary Oil and Gas Information - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Prices crude oil	$ 81.33	$ 83.79
Prices natural gas	5.46	5.84
Annual discount rate	10.00%
Benchmark
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Prices crude oil	94.71	96.19
Prices natural gas	$ 2.76	$ 4.12