Derivative Instruments Related to Natural Gas Production and Processing Activities Table (Detail) (Natural Gas [Member])	Sep. 30, 2011
Contracted Commodities in 2011 [Member] \| Three-Way Collars [Member]
Derivative [Line Items]
Nonmonetary notional amount of price risk derivative instruments not designated as hedging instruments	480
Contracted Commodities in 2011 [Member] \| Three-Way Collars [Member] \| Call Options Sold [Member]
Average price per MMBtu
Average ceiling price	8.29
Contracted Commodities in 2011 [Member] \| Three-Way Collars [Member] \| Put Options Purchased [Member]
Average price per MMBtu
Average floor price	6.50
Contracted Commodities in 2011 [Member] \| Three-Way Collars [Member] \| Put Options Sold [Member]
Average price per MMBtu
Average floor price	5.00
Contracted Commodities in 2011 [Member] \| Fixed-Price Contracts [Member]
Derivative [Line Items]
Nonmonetary notional amount of price risk derivative instruments not designated as hedging instruments	90
Average price per MMBtu
Average price per MMBtu	6.17
Contracted Commodities in 2011 [Member] \| Basis Swaps [Member]
Derivative [Line Items]
Nonmonetary notional amount of price risk derivative instruments not designated as hedging instruments	45
Average price per MMBtu
Average price per MMBtu	(1.74)
Contracted Commodities in 2012 [Member] \| Three-Way Collars [Member]
Derivative [Line Items]
Nonmonetary notional amount of price risk derivative instruments not designated as hedging instruments	500
Contracted Commodities in 2012 [Member] \| Three-Way Collars [Member] \| Call Options Sold [Member]
Average price per MMBtu
Average ceiling price	9.03
Contracted Commodities in 2012 [Member] \| Three-Way Collars [Member] \| Put Options Purchased [Member]
Average price per MMBtu
Average floor price	6.50
Contracted Commodities in 2012 [Member] \| Three-Way Collars [Member] \| Put Options Sold [Member]
Average price per MMBtu
Average floor price	5.00
Contracted Commodities in 2013 [Member] \| Three-Way Collars [Member]
Derivative [Line Items]
Nonmonetary notional amount of price risk derivative instruments not designated as hedging instruments	450
Contracted Commodities in 2013 [Member] \| Three-Way Collars [Member] \| Call Options Sold [Member]
Average price per MMBtu
Average ceiling price	6.57
Contracted Commodities in 2013 [Member] \| Three-Way Collars [Member] \| Put Options Purchased [Member]
Average price per MMBtu
Average floor price	5.00
Contracted Commodities in 2013 [Member] \| Three-Way Collars [Member] \| Put Options Sold [Member]
Average price per MMBtu
Average floor price	4.00

Average price per MMBtu or barrel [Abstract]

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- Definition

The average price on instruments used to modify the payment/receipt from one variable rate to another.

+ Details

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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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- Definition

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.

+ Details

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- Definition

The average forward price on the group of price risk forward or futures contracts, such as gas futures contracts.

+ Details

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

Aggregate notional amount of price risk derivatives not designated as a hedging instrument with notional amounts expressed in nonmonetary units. For example the number of barrels specified in a fuel oil forward purchase contract.

+ References

Reference 1: 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

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Notional Amount

-URI http://asc.fasb.org/extlink&oid=6519104

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 1A

-Subparagraph (d)

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

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