Derivative Instruments - Schedule of Outstanding Commodity Derivative Instruments (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2012 Barrels
Fair Value Asset (Liability)	$ 22,569
2013 Two-way collar [Member]
Total Notional Amount of Oil (Barrels)	2,116,000
Average Floor Price	87.05
Average Ceiling Price	98.78
Fair Value Asset (Liability)	56
Settlement Period	2013
2013 Three-way collar [Member]
Total Notional Amount of Oil (Barrels)	2,357,420
Average Sub-Floor Price	$ 65.96
Average Floor Price	92.16
Average Ceiling Price	111.26
Fair Value Asset (Liability)	8,603
Settlement Period	2013
2013 Put Spreads [Member]
Total Notional Amount of Oil (Barrels)	1,717,080
Average Sub-Floor Price	$ 70.71
Average Floor Price	91.24
Fair Value Asset (Liability)	7,567
Settlement Period	2013
2013 Swaps [Member]
Total Notional Amount of Oil (Barrels)	1,033,000
Average Swap Price	94.76
Fair Value Asset (Liability)	1,742
Settlement Period	2013
2014 Two-way collar [Member]
Total Notional Amount of Oil (Barrels)	170,500
Average Floor Price	86.82
Average Ceiling Price	97.75
Fair Value Asset (Liability)	(69)
Settlement Period	2014
2014 Three-way collar [Member]
Total Notional Amount of Oil (Barrels)	2,361,030
Average Sub-Floor Price	$ 70.38
Average Floor Price	90.90
Average Ceiling Price	106.88
Fair Value Asset (Liability)	3,453
Settlement Period	2014
2014 Put Spreads [Member]
Total Notional Amount of Oil (Barrels)	150,970
Average Sub-Floor Price	$ 71.03
Average Floor Price	91.03
Fair Value Asset (Liability)	889
Settlement Period	2014
2014 Swaps [Member]
Total Notional Amount of Oil (Barrels)	93,000
Average Swap Price	94.76
Fair Value Asset (Liability)	154
Settlement Period	2014
2015 Three-way collars [Member]
Total Notional Amount of Oil (Barrels)	201,500
Average Sub-Floor Price	$ 70.77
Average Floor Price	90.77
Average Ceiling Price	106.48
Fair Value Asset (Liability)	$ 174
Settlement Period	2015

Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract.

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 12

-Section 13

-Sentence Column B

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- Definition

The average strike price of the sold put that establishes the price at which if the market price falls below, the minimum price would be index price plus the difference between the purchased put (floor) and the sold put strike price.

+ References

No definition available.

+ Details

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- Definition

Derivatives, Maturity Date

+ References

No definition available.

+ 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.

+ References

No definition available.

+ 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.

+ References

No definition available.

+ Details

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- Definition

Fair value of the assets less the liabilities of a derivative or group of derivatives.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 45

-Paragraph 6

-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13433-108611

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 45

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 15

-URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13495-108611

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- Definition

Average fixed price related to the group of price risk swap derivatives.

+ References

No definition available.

+ Details

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