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Supplemental Information on Oil and Natural Gas Producing Activities - Summary of Changes in Estimated Oil and Natural Gas Reserves (Unaudited) (Details)

12 Months Ended

Dec. 31, 2012

MBbls

Dec. 31, 2011

MBbls

Dec. 31, 2010

MBbls

Dec. 31, 2009

MBbls

Oil Reserves

Proved Developed and Undeveloped Reserves

Proved developed and undeveloped reserves, beginning balance (in MBbls for Oil and NGLs/MMcf for Natural Gas)

244,785

^[1]

252,114

105,349

Revisions of previous estimates (in MBbls for Oil and NGLs/MMcf for Natural Gas)

(9,278)

12,999

Acquisitions of new reserves (in MBbls for Oil and NGLs/MMcf for Natural Gas)

1,533

71,640

Extensions and discoveries (in MBbls for Oil and NGLs/MMcf for Natural Gas)

55,577

69,512

Sales of reserves in place (in MBbls for Oil and NGLs/MMcf for Natural Gas)

23,600

(43,331)

0

Production (in MBbls for Oil and NGLs/MMcf for Natural Gas)

(11,830)

(7,386)

Proved developed and undeveloped reserves, ending balance (in MBbls for Oil and NGLs/MMcf for Natural Gas)

244,785

^[1]

252,114

Proved developed reserves (in MBbls for Oil and NGLs/MMcf for Natural Gas)

118,728

91,965

38,327

Proved undeveloped reserves (in MBbls for Oil and NGLs/MMcf for Natural Gas)

126,057

160,149

67,022

Oil Excluding NGLs

Proved Developed and Undeveloped Reserves

Proved developed and undeveloped reserves, beginning balance (in MBbls for Oil and NGLs/MMcf for Natural Gas)

214,450

^[1]

Revisions of previous estimates (in MBbls for Oil and NGLs/MMcf for Natural Gas)

(37,394)

Acquisitions of new reserves (in MBbls for Oil and NGLs/MMcf for Natural Gas)

31,470

Extensions and discoveries (in MBbls for Oil and NGLs/MMcf for Natural Gas)

89,656

Sales of reserves in place (in MBbls for Oil and NGLs/MMcf for Natural Gas)

(20,269)

Production (in MBbls for Oil and NGLs/MMcf for Natural Gas)

(15,868)

Proved developed and undeveloped reserves, ending balance (in MBbls for Oil and NGLs/MMcf for Natural Gas)

262,045

Proved developed reserves (in MBbls for Oil and NGLs/MMcf for Natural Gas)

136,605

Proved undeveloped reserves (in MBbls for Oil and NGLs/MMcf for Natural Gas)

125,440

Natural Gas Liquids

Proved Developed and Undeveloped Reserves

Proved developed and undeveloped reserves, beginning balance (in MBbls for Oil and NGLs/MMcf for Natural Gas)

30,335

^[1],[2]

Revisions of previous estimates (in MBbls for Oil and NGLs/MMcf for Natural Gas)

15,098

^[2]

Acquisitions of new reserves (in MBbls for Oil and NGLs/MMcf for Natural Gas)

683

^[2]

Extensions and discoveries (in MBbls for Oil and NGLs/MMcf for Natural Gas)

27,259

^[2]

Sales of reserves in place (in MBbls for Oil and NGLs/MMcf for Natural Gas)

(3,287)

^[2]

Production (in MBbls for Oil and NGLs/MMcf for Natural Gas)

(2,094)

^[2]

Proved developed and undeveloped reserves, ending balance (in MBbls for Oil and NGLs/MMcf for Natural Gas)

67,994

^[1],[2]

Proved developed reserves (in MBbls for Oil and NGLs/MMcf for Natural Gas)

33,785

^[2]

Proved undeveloped reserves (in MBbls for Oil and NGLs/MMcf for Natural Gas)

34,209

^[2]

Natural Gas Reserves

Proved Developed and Undeveloped Reserves

Proved developed and undeveloped reserves, beginning balance (in MBbls for Oil and NGLs/MMcf for Natural Gas)

1,355,056

^[1],[3]

1,762,665

^[3]

680,075

^[3]

Revisions of previous estimates (in MBbls for Oil and NGLs/MMcf for Natural Gas)

(538,214)

^[3]

(164,845)

^[3]

867,931

^[3]

Acquisitions of new reserves (in MBbls for Oil and NGLs/MMcf for Natural Gas)

202,995

^[3]

2,906

^[3]

79,942

^[3]

Extensions and discoveries (in MBbls for Oil and NGLs/MMcf for Natural Gas)

489,302

^[3]

299,848

^[3]

211,150

^[3]

Sales of reserves in place (in MBbls for Oil and NGLs/MMcf for Natural Gas)

(548)

^[3]

(476,212)

^[3]

(207)

^[3]

Production (in MBbls for Oil and NGLs/MMcf for Natural Gas)

(93,549)

^[3]

(69,306)

^[3]

(76,226)

^[3]

Proved developed and undeveloped reserves, ending balance (in MBbls for Oil and NGLs/MMcf for Natural Gas)

1,415,042

^[1],[3]

1,355,056

^[1],[3]

1,762,665

^[3]

Proved developed reserves (in MBbls for Oil and NGLs/MMcf for Natural Gas)

896,701

^[3]

670,382

^[3]

784,292

^[3]

592,777

^[3]

Proved undeveloped reserves (in MBbls for Oil and NGLs/MMcf for Natural Gas)

518,341

^[3]

684,674

^[3]

978,373

^[3]

87,298

^[3]

[1]	Proved reserves at December 31, 2012 and 2011, respectively, include approximately 17,340 MBbls and 17,018 MBbls of oil, 5,132 MBbls and 1,782 MBbls of NGLs and 94,543 MMcf and 45,500 MMcf of natural gas attributable to noncontrolling interests.
[2]	Prior to 2012, NGLs did not comprise a significant portion of total proved reserves and were included with oil reserves, which affects the comparability of estimated reserves for 2012, 2011 and 2010.
[3]	Natural gas reserves are computed at 14.65 pounds per square inch absolute and 60 degrees Fahrenheit.

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Additions to proved reserves that result from (1) extension of the proved acreage of previously discovered (old) reservoirs through additional drilling in periods after discovery and (2) discovery of new fields with proved reserves or of new reservoirs of proved reserves in old fields.

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The net quantity of proved reserves as of the balance sheet date. Proved oil reserves are the estimated quantities of crude oil and natural gas liquids which geological and engineering data demonstrate with reasonable certainty to be recoverable.

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Net quantities of an enterprise's interests in proved developed reserves of either crude oil (including condensate and natural gas liquids), natural gas, synthetic oil and gas, or other nonrenewable 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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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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