Annual report pursuant to Section 13 and 15(d)

Supplemental Information On Oil And Gas Producing Activities (Changes In Standardized Measure Of Discounted Future Net Cash Flows) (Detail)

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Supplemental Information On Oil And Gas Producing Activities (Changes In Standardized Measure Of Discounted Future Net Cash Flows) (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Changes in Standardized Measure of Discounted Future Net Cash Flows:      
Balance at Beginning of Period $ 149,387 $ 137,436 $ 152,902
Sales of oil and natural gas, net of production costs (40,349) (95,973) (132,662)
Net changes in prices and production costs (146,536) (28,098) (52,056)
Revisions of previous quantity estimates (104,158) 74,497 43,815
Additions   2,188 29,620
Changes in estimated future development costs (15,604) 31,686 (5,345)
Development costs incurred during the period 60,004   44,389
Accretion of discount 27,312 24,163 15,290
Net change of income taxes 104,303 (15,609) 26,120
Change in production rates (timing) and other (7,218) 19,097 15,363
Balance at End of Period $ 27,141 $ 149,387 $ 137,436