Commodity Derivative Positions (Detail) (Commodity contracts, USD $)	9 Months Ended
Sep. 30, 2011 MMBtu
Derivative Instruments Related to Oil and Gas Production [Line Items]
Fair Value Asset	$ 19,844,000
Fair Value Liability
Natural Gas \| Group Three
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Fourth quarter 2011
Average Volume Per Day	20,000
Weighted Average Price Floor/Swap	6.00
Weighted Average Price Ceiling	8.50
Fair Value Asset	4,047,000
Fair Value Liability
Instrument Type	Costless collars
Natural Gas \| Group Four
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	First quarter 2012
Average Volume Per Day	20,000
Weighted Average Price Floor/Swap	6.00
Weighted Average Price Ceiling	8.50
Fair Value Asset	3,405,000
Fair Value Liability
Instrument Type	Costless collars
Natural Gas \| Group Five
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Fourth quarter 2011
Average Volume Per Day	10,000
Weighted Average Price Floor/Swap	5.01
Fair Value Asset	1,114,000
Fair Value Liability
Instrument Type	Swaps
Natural Gas \| Group Six
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	First quarter 2012
Average Volume Per Day	10,000
Weighted Average Price Floor/Swap	5.10
Fair Value Asset	881,000
Fair Value Liability
Instrument Type	Swaps
Natural Gas \| Group Seven
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Second quarter 2012
Average Volume Per Day	20,000
Weighted Average Price Floor/Swap	5.31
Fair Value Asset	2,117,000
Fair Value Liability
Instrument Type	Swaps
Natural Gas \| Group Eight
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Third quarter 2012
Average Volume Per Day	20,000
Weighted Average Price Floor/Swap	5.31
Fair Value Asset	1,952,000
Fair Value Liability
Instrument Type	Swaps
Natural Gas \| Group Nine
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Fourth quarter 2012
Average Volume Per Day	10,000
Weighted Average Price Floor/Swap	5.10
Fair Value Asset	557,000
Fair Value Liability
Instrument Type	Swaps
Crude Oil \| Group Ten
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Fourth quarter 2011
Average Volume Per Day	360
Weighted Average Price Floor/Swap	80.00
Weighted Average Price Ceiling	103.30
Fair Value Asset	168,000
Fair Value Liability
Instrument Type	Costless collars
Crude Oil \| Group 11
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	First quarter 2012
Average Volume Per Day	500
Weighted Average Price Floor/Swap	100.00
Weighted Average Price Ceiling	120.00
Fair Value Asset	983,000
Fair Value Liability
Instrument Type	Costless collars
Crude Oil \| Group 12
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Second quarter 2012
Average Volume Per Day	500
Weighted Average Price Floor/Swap	100.00
Weighted Average Price Ceiling	120.00
Fair Value Asset	986,000
Fair Value Liability
Instrument Type	Costless collars
Crude Oil \| Group 13
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Third quarter 2012
Average Volume Per Day	500
Weighted Average Price Floor/Swap	100.00
Weighted Average Price Ceiling	120.00
Fair Value Asset	974,000
Fair Value Liability
Instrument Type	Costless collars
Crude Oil \| Group 14
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Fourth quarter 2012
Average Volume Per Day	500
Weighted Average Price Floor/Swap	100.00
Weighted Average Price Ceiling	120.00
Fair Value Asset	951,000
Fair Value Liability
Instrument Type	Costless collars
Crude Oil \| Group 15
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Fourth quarter 2011
Average Volume Per Day	500
Weighted Average Price Floor/Swap	109.00
Fair Value Asset	1,709,000
Fair Value Liability
Instrument Type	Swaps

Derivative, average volume per day

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- Definition

Derivative Instruments Related to Oil and Gas Production [Line Items]

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- Definition

The average cap rate on a group of price risk derivatives such as caps or collars. A payment or receipt is triggered if the market rate exceeds the cap rate on the contract.

+ Details

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The average floor rate on a group of price risk derivatives such as floors or collars. A payment or receipt is triggered if the market rate falls below the floor rate on the contract.

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- Definition

Fair value as of the balance sheet date of derivative instrument not designated as hedging instrument and classified as an asset.

+ References

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.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4B

-Subparagraph (c)(2)

-URI http://asc.fasb.org/extlink&oid=6935481&loc=SL5624163-113959

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- Definition

Fair value as of the balance sheet date of derivative instrument not designated as hedging instrument and classified as a liability.

+ References

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- Definition

The time remaining until the derivative matures.

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Description of the type of derivative instrument, such as a swap, forward, and option. Excludes the underlying risk of the instrument.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6935481&loc=SL5579240-113959

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 1A

-URI http://asc.fasb.org/extlink&oid=6935481&loc=SL5579245-113959

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(n)(2))

-URI http://asc.fasb.org/extlink&oid=6881521&loc=d3e23780-122690

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 1B

-URI http://asc.fasb.org/extlink&oid=6935481&loc=SL5580258-113959

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