Oil and Gas Reserves Information (Unaudited) - Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Reserves (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Cash Flows Relating to Proved Reserves:
Future Cash Flows	$ 3,817,982	$ 3,053,078
Future Costs:
Production	(1,307,923)	(915,053)
Development and Abandonment	(649,758)	(544,084)
Future Income Taxes	(451,708)	(372,813)
Future Net Cash Flows	1,408,593	1,221,128
10% Discount Factor	(601,376)	(579,803)
Standardized Measure of Discounted Future Net Cash Flows	$ 807,217	$ 641,325