Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Assets:
Commodity derivative assets – current	$ 97,956	$ 128,981
Derivative assets	0	35,897
Fair Value, Measurements, Recurring
Assets:
Assets of SERP	4,123	4
Liabilities:
Deferred compensation – SERP obligation	(4,125)	(4,178)
Fair Value, Measurements, Recurring | Level 1
Assets:
Assets of SERP	4,123	4
Liabilities:
Deferred compensation – SERP obligation	(4,125)	(4,178)
Fair Value, Measurements, Recurring | Level 2
Assets:
Assets of SERP	0	0
Liabilities:
Deferred compensation – SERP obligation	0	0
Fair Value, Measurements, Recurring | Level 3
Assets:
Assets of SERP	0	0
Liabilities:
Deferred compensation – SERP obligation	0	0
Fair Value, Measurements, Recurring | Commodity contracts
Assets:
Commodity derivative assets – current	97,956	128,981
Derivative assets		35,897
Fair Value, Measurements, Recurring | Commodity contracts | Level 1
Assets:
Commodity derivative assets – current	0	0
Derivative assets		0
Fair Value, Measurements, Recurring | Commodity contracts | Level 2
Assets:
Commodity derivative assets – current	97,956	128,981
Derivative assets		35,897
Fair Value, Measurements, Recurring | Commodity contracts | Level 3
Assets:
Commodity derivative assets – current	$ 0	0
Derivative assets		$ 0