Oil and Gas Reserves Information (Unaudited) - Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Reserves (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash Flows Relating to Proved Reserves:
Future Cash Flows	$ 3,588,764	$ 2,267,877
Future Costs:
Production	(986,398)	(798,454)
Development and Abandonment	(672,559)	(502,848)
Future Income Taxes	5,239	(6,488)
Future Net Cash Flows	1,935,046	960,087
10% Discount Factor	(1,053,502)	(530,812)
Standardized Measure of Discounted Future Net Cash Flows	881,544	429,275	$ 372,139	$ 1,090,660
Assets Held for Sale
Future Costs:
Standardized Measure of Discounted Future Net Cash Flows	$ 109,134	$ 77,146