Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Assets:
Commodity derivative assets – current	$ 128,981us-gaap_DerivativeAssetsCurrent	$ 3,830us-gaap_DerivativeAssetsCurrent
Commodity derivative assets – noncurrent	35,897us-gaap_DerivativeAssetsNoncurrent	1,552us-gaap_DerivativeAssetsNoncurrent
Liabilities:
Commodity derivative liabilities – noncurrent	0us-gaap_DerivativeLiabilitiesNoncurrent	0us-gaap_DerivativeLiabilitiesNoncurrent
Fair Value, Measurements, Recurring
Assets:
Assets of SERP	4,123	3,734
Liabilities:
Deferred compensation – SERP obligation and other	(4,178)	(3,879)
Fair Value, Measurements, Recurring \| Level 1
Assets:
Assets of SERP	4,123	3,734
Liabilities:
Deferred compensation – SERP obligation and other	(4,178)	(3,879)
Fair Value, Measurements, Recurring \| Level 2
Assets:
Assets of SERP	0	0
Liabilities:
Deferred compensation – SERP obligation and other	0	0
Fair Value, Measurements, Recurring \| Level 3
Assets:
Assets of SERP	0	0
Liabilities:
Deferred compensation – SERP obligation and other	0	0
Fair Value, Measurements, Recurring \| Commodity contracts
Assets:
Commodity derivative assets – current	128,981	3,830
Commodity derivative assets – noncurrent	35,897	1,552
Liabilities:
Commodity derivative liabilities – noncurrent		(10,141)
Fair Value, Measurements, Recurring \| Commodity contracts \| Level 1
Assets:
Commodity derivative assets – current	0	0
Commodity derivative assets – noncurrent	0	0
Liabilities:
Commodity derivative liabilities – noncurrent		0
Fair Value, Measurements, Recurring \| Commodity contracts \| Level 2
Assets:
Commodity derivative assets – current	128,981	3,830
Commodity derivative assets – noncurrent	35,897	1,552
Liabilities:
Commodity derivative liabilities – noncurrent		(10,141)
Fair Value, Measurements, Recurring \| Commodity contracts \| Level 3
Assets:
Commodity derivative assets – current	0	0
Commodity derivative assets – noncurrent	0	0
Liabilities:
Commodity derivative liabilities – noncurrent		$ 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.

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- 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 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.

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