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Supplemental Oil and Natural Gas Disclosures (Unaudited) - Costs Incurred in Oil and Natural Gas Property Acquisition, Exploration and Development (Detail) (Consolidated Entities [Member], USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Consolidated Entities [Member]
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Acquisition of properties	$ 4,418	$ 714	$ 0
Exploration costs	1,752	549	0
Development costs	15,938	3,597	5
Total cost incurred for consolidated entities	$ 22,108	$ 4,860	$ 5

Cost incurred, including capitalized costs and costs charged to expense, in acquisition of oil and gas properties.

Development costs incurred, including capitalized costs and costs charged to expense, in oil and gas activities.

Exploration costs incurred, including capitalized costs and costs charged to expense, in oil and gas activities.