Derivative Activities - Fair Value of Derivative Contracts, by Consolidated Balance Sheets (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Derivatives, Fair Value [Line Items]
Assets Carrying Value	$ 152,114	$ 282,546
Liabilities Carrying Value	(12,963)	(2,373)
Net Carrying Value	139,151	280,173
Asset Carrying Value	15,086	17,949
Liabilities Carrying Value	(8,904)	(46,795)
Net Carrying Value	6,182	(28,846)
Sold swaps [Member]
Derivatives, Fair Value [Line Items]
Asset Carrying Value	7,316	17,949
Liabilities Carrying Value	(8,904)	(2,794)
Net Carrying Value	(1,588)	15,155
Re-purchased swaps [Member]
Derivatives, Fair Value [Line Items]
Asset Carrying Value	5,920
Net Carrying Value	5,920
Collars [Member]
Derivatives, Fair Value [Line Items]
Assets Carrying Value	129,878	228,228
Liabilities Carrying Value	(9,721)	(1,954)
Net Carrying Value	120,157	226,274
Asset Carrying Value	857
Liabilities Carrying Value		(14,653)
Net Carrying Value	857	(14,653)
Call options [Member]
Derivatives, Fair Value [Line Items]
Liabilities Carrying Value		(29,348)
Net Carrying Value		(29,348)
Basis Swaps [Member]
Derivatives, Fair Value [Line Items]
Asset Carrying Value	993
Net Carrying Value	993
Swaps [Member]
Derivatives, Fair Value [Line Items]
Assets Carrying Value	22,236	54,318
Liabilities Carrying Value	(3,242)	(419)
Net Carrying Value	$ 18,994	$ 53,899

Fair value of all asset derivatives designated as cash flow hedging instruments.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4B

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 107

-Paragraph 10

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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Fair value of all liability derivatives designated as cash flow hedging instruments.

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Net fair value of all derivative instruments designated as cash flow hedging instruments, which is ordinarily reflected within other comprehensive income.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 107

-Paragraph 10

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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Fair value of all derivative assets not designated as a hedging instrument.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 10

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