Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Assets:
Commodity derivative assets – current	$ 80,372	$ 128,981
Commodity derivative assets – noncurrent	19,546	35,897
Liabilities:
Commodity derivative liabilities – current	0	0
Fair Value, Measurements, Recurring | Estimate of Fair Value, Fair Value Disclosure [Member]
Assets:
Assets of SERP	4,206	4,123
Liabilities:
Deferred compensation – SERP obligations	(4,242)	(4,178)
Fair Value, Measurements, Recurring | Level 1
Assets:
Assets of SERP	4,206	4,123
Liabilities:
Deferred compensation – SERP obligations	(4,242)	(4,178)
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 | Estimate of Fair Value, Fair Value Disclosure [Member]
Assets:
Commodity derivative assets – current	80,372	128,981
Commodity derivative assets – noncurrent	19,546	35,897
Fair Value, Measurements, Recurring | Commodity contracts | Level 1
Assets:
Commodity derivative assets – current	0	0
Commodity derivative assets – noncurrent	0	0
Fair Value, Measurements, Recurring | Commodity contracts | Level 2
Assets:
Commodity derivative assets – current	80,372	128,981
Commodity derivative assets – noncurrent	19,546	35,897
Fair Value, Measurements, Recurring | Commodity contracts | Level 3
Assets:
Commodity derivative assets – current	0	0
Commodity derivative assets – noncurrent	$ 0	$ 0