Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Assets:
Commodity derivative assets – current	$ 118,409us-gaap_DerivativeAssetsCurrent	$ 128,981us-gaap_DerivativeAssetsCurrent
Commodity derivative assets – noncurrent	31,844us-gaap_DerivativeAssetsNoncurrent	35,897us-gaap_DerivativeAssetsNoncurrent
Liabilities:
Commodity derivative liabilities – current	0us-gaap_DerivativeLiabilitiesCurrent	0us-gaap_DerivativeLiabilitiesCurrent
Fair Value, Measurements, Recurring \| Estimate of Fair Value, Fair Value Disclosure [Member]
Assets:
Assets of SERP	4,216	4,123
Liabilities:
Deferred compensation – SERP obligations	(4,269)	(4,178)
Fair Value, Measurements, Recurring \| Level 1
Assets:
Assets of SERP	4,216	4,123
Liabilities:
Deferred compensation – SERP obligations	(4,269)	(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	118,409	128,981
Commodity derivative assets – noncurrent	31,844	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	118,409	128,981
Commodity derivative assets – noncurrent	31,844	35,897
Fair Value, Measurements, Recurring \| Commodity contracts \| Level 3
Assets:
Commodity derivative assets – current	0	0
Commodity derivative assets – noncurrent	$ 0	$ 0

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, expected to be settled within one year or normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, expected to be settled after one year or the normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References+ Details

- Definition

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, expected to be settled within one year or normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References+ Details