Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Assets:
Commodity derivative assets – current	$ 49,030	$ 97,956
Fair Value, Measurements, Recurring
Assets:
Assets of SERP	4,154	4,123
Liabilities:
Deferred compensation – SERP obligations	(4,155)	(4,125)
Fair Value, Measurements, Recurring | Level 1
Assets:
Assets of SERP	4,154	4,123
Liabilities:
Deferred compensation – SERP obligations	(4,155)	(4,125)
Fair Value, Measurements, Recurring | Level 2
Assets:
Assets of SERP	0	0
Liabilities:
Deferred compensation – SERP obligations	0	0
Fair Value, Measurements, Recurring | Level 3
Assets:
Assets of SERP	0	0
Liabilities:
Deferred compensation – SERP obligations	0	0
Fair Value, Measurements, Recurring | Commodity contracts
Assets:
Commodity derivative assets – current	49,030	97,956
Fair Value, Measurements, Recurring | Commodity contracts | Level 1
Assets:
Commodity derivative assets – current	0	0
Fair Value, Measurements, Recurring | Commodity contracts | Level 2
Assets:
Commodity derivative assets – current	49,030	97,956
Fair Value, Measurements, Recurring | Commodity contracts | Level 3
Assets:
Commodity derivative assets – current	$ 0	$ 0