Commitments and Contingencies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2013
Wesley West Minerals, Ltd and Longfellow Ranch Partners, LP
Commitments and Contingencies Disclosure [Line Items]
Damages sought by plaintiff	$ 45.5
General Land Office of the State of Texas
Commitments and Contingencies Disclosure [Line Items]
Damages sought by plaintiff	13.0
Patriot Exploration, LLC, Jonathan Feldman, Redwing Drilling Partners, Mapleleaf Drilling Partners, Avalanche Drilling Partners, Penguin Drilling Partners and Gramax Insurance Company Ltd.
Commitments and Contingencies Disclosure [Line Items]
Plaintiffs investment under participation agreement	16.0
Treating Agreement
Commitments and Contingencies Disclosure [Line Items]
Contract agreement, term	30 years
Minimum delivery required (in Bcf)	3,200
Treating Agreement \| Minimum
Commitments and Contingencies Disclosure [Line Items]
Range of projected shortfall in one year	29.5
Treating Agreement \| Maximum
Commitments and Contingencies Disclosure [Line Items]
Range of projected shortfall in one year	$ 36.0

The value (monetary amount) of the award the plaintiff seeks in the legal matter.

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