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Supplemental Information on Natural Gas and Oil Exploration, Development and Production Activities (Unaudited) - Changes in Proved Undeveloped Reserves (Parenthetical) (Detail) - MMcfe
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Extractive Industries [Abstract]    
Proved undeveloped reserves dropped 1,200,000 611,000