Supplemental Information on Natural Gas and Oil Exploration, Development and Production Activities (Unaudited) - Changes in Proved Undeveloped Reserves for 2012 (Mmcfe) (Detail)	12 Months Ended
Dec. 31, 2012 MMcf
Reserve Quantities [Line Items]
Beginning proved undeveloped reserves-2011	2,652,687
Undeveloped reserves transferred to developed	(412,502)
Revisions	(119,430)
Purchases/sales
Extension and discoveries	927,313
Ending proved undeveloped reserves-2012	3,048,068

- Definition

Net quantities of an enterprise's interests in proved undeveloped reserves of either crude oil (including condensate and natural gas liquids), natural gas, synthetic oil and gas, or other renewable natural resource that is intended to be upgraded into synthetic oil and gas as of the beginning and the end of the year. "Net" quantities of reserves include those relating to the enterprise's operating and nonoperating interests in properties. Quantities of reserves relating to royalty interests owned are included in "net" quantities if the necessary information is available to the enterprise. "Net" quantities does not include reserves relating to interests of others in properties owned by the enterprise. The unit of measure for reserve quantities is defined as "barrels" for oil and synthetic oil reserves or "cubic feet" or "cubic meters" for natural gas and synthetic gas reserves.

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