Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Assets:
Commodity derivative assets - current	$ 4,424	$ 11,292
Commodity derivative assets - noncurrent	1,900	5,181
Liabilities:
Commodity derivative liabilities - current	(15,305)	0
Commodity derivative liabilities - noncurrent	(799)	(1,421)
Fair Value, Measurements, Recurring \| Estimate of Fair Value, Fair Value Disclosure [Member]
Assets:
Assets of SERP	3,537	3,237
Liabilities:
Deferred compensation - SERP obligations	(3,639)	(3,305)
Fair Value, Measurements, Recurring \| Level 1
Assets:
Assets of SERP	3,537	3,237
Liabilities:
Deferred compensation - SERP obligations	(3,639)	(3,305)
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	4,424	11,292
Commodity derivative assets - noncurrent	1,900	5,181
Liabilities:
Commodity derivative liabilities - current	(15,305)
Commodity derivative liabilities - noncurrent	(799)	(1,421)
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	4,424	11,292
Commodity derivative assets - noncurrent	1,900	5,181
Liabilities:
Commodity derivative liabilities - current	(15,305)
Commodity derivative liabilities - noncurrent	(799)	(1,421)
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 values as of the balance sheet date for all assets resulting from contracts that meet the criteria of being accounted for as derivative instruments and which are expected to be converted into cash or otherwise disposed of within a year or the normal operating cycle, if longer, net of the effects of master netting arrangements.

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+ Details

- Definition

Fair values as of the balance sheet date of all assets resulting from contracts that meet the criteria of being accounted for as derivative instruments which are expected to exist longer than one year or beyond the normal operating cycle, if longer, net of the effects of master netting arrangements.

+ References

+ Details

- Definition

Fair values as of the balance sheet date of all liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments, and which are expected to be extinguished or otherwise disposed of within a year or the normal operating cycle, if longer, net of the effects of master netting arrangements.

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Fair values as of the balance sheet date of all liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments, and which are expected to be extinguished or otherwise disposed of after one year or beyond the normal operating cycle, if longer, net of the effects of master netting arrangements.

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+ Details