Supplemental Information on Oil and Gas Producing Activities (Capitalized Costs Relating to Oil and Gas Producing Activities) (Detail) - USD ($) $ in Thousands		Dec. 31, 2015	Dec. 31, 2014
Capitalized Costs Relating to Oil and Gas Producing Activities:
Properties not being amortized		$ 10,000	$ 47,290
Properties being amortized	[1]	423,541	347,186
Total capitalized costs		433,541	394,476
Less accumulated depreciation, depletion, and amortization		(400,168)	(289,272)
Net capitalized costs		33,373	105,204
Asset retirement cost		$ 8,700	$ 5,200

[1] Includes $8.7 million and $5.2 million asset retirement cost in 2015 and 2014.