Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Assets:
Commodity derivative assets - current	$ 65	$ 3,830
Commodity derivative assets - noncurrent	1,974	1,552
Liabilities:
Commodity derivative liabilities - current	(19,403)	(10,141)
Commodity derivative liabilities - noncurrent	0	0
Fair Value, Measurements, Recurring \| Estimate of Fair Value, Fair Value Disclosure [Member]
Assets:
Assets of SERP	3,815	3,734
Liabilities:
Deferred compensation - SERP obligations	(4,084)	(3,879)
Fair Value, Measurements, Recurring \| Level 1
Assets:
Assets of SERP	3,815	3,734
Liabilities:
Deferred compensation - SERP obligations	(4,084)	(3,879)
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	65	3,830
Commodity derivative assets - noncurrent	1,974	1,552
Liabilities:
Commodity derivative liabilities - current	(19,403)
Commodity derivative liabilities - noncurrent	0	(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 - current	0
Commodity derivative liabilities - noncurrent	0	0
Fair Value, Measurements, Recurring \| Commodity contracts \| Level 2
Assets:
Commodity derivative assets - current	65	3,830
Commodity derivative assets - noncurrent	1,974	1,552
Liabilities:
Commodity derivative liabilities - current	(19,403)
Commodity derivative liabilities - noncurrent	0	(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 - current	0
Commodity derivative liabilities - 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.

+ References+ Details

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

- 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.

+ References+ Details