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Supplemental Oil and Gas Reserve Information - Net Proved, Proved Developed and Proved Undeveloped Reserves (Detail) | 12 Months Ended
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Dec. 31, 2012 MBbls | Dec. 31, 2011 MBbls | Dec. 31, 2010 MBbls
Reserve Quantities [Line Items] |  |  |
Extensions, discoveries and other additions | 73,871 | 43,168 | 24,473
Gas MMcf [Member] |  |  |
Reserve Quantities [Line Items] |  |  |
Proved reserves, beginning balance | 57,900 | 19,379 | 5,270
Revisions of previous estimates | (8,495) | (159) | 1,897
Extensions, discoveries and other additions | 45,759 | 40,220 | 12,172
Sales of reserves in place |  | (518) | (5)
Purchases of reserves in place | 512 | 65 | 696
Production | (4,146) | (1,087) | (651)
Net proved reserves, closing balance | 91,530 | 57,900 | 19,379
Gas MMcf [Member] | Proved developed reserves [Member] |  |  |
Reserve Quantities [Line Items] |  |  |
Net proved reserves, closing balance | 44,695 | 24,535 | 8,208
Gas MMcf [Member] | Proved undeveloped reserves [Member] |  |  |
Reserve Quantities [Line Items] |  |  |
Net proved reserves, closing balance | 46,835 | 33,365 | 11,171
Oil MBbl [Member] |  |  |
Reserve Quantities [Line Items] |  |  |
Proved reserves, beginning balance | 69,064 | 36,550 | 12,421
Revisions of previous estimates | (567) | (262) | 2,235
Extensions, discoveries and other additions | 66,245 | 36,464 | 22,445
Sales of reserves in place |  | (56) | (122)
Purchases of reserves in place | 881 | 100 | 1,363
Production | (7,533) | (3,732) | (1,792)
Net proved reserves, closing balance | 128,090 | 69,064 | 36,550
Oil MBbl [Member] | Proved developed reserves [Member] |  |  |
Reserve Quantities [Line Items] |  |  |
Net proved reserves, closing balance | 62,602 | 31,749 | 15,650
Oil MBbl [Member] | Proved undeveloped reserves [Member] |  |  |
Reserve Quantities [Line Items] |  |  |
Net proved reserves, closing balance | 65,488 | 37,315 | 20,900
MBoe [Member] |  |  |
Reserve Quantities [Line Items] |  |  |
Proved reserves, beginning balance | 78,715 | 39,780 | 13,299
Revisions of previous estimates | (1,983) | (288) | 2,552
Extensions, discoveries and other additions | 73,871 | 43,168 | 24,473
Sales of reserves in place |  | (142) | (123)
Purchases of reserves in place | 966 | 111 | 1,479
Production | (8,224) | (3,914) | (1,900)
Net proved reserves, closing balance | 143,345 | 78,715 | 39,780
MBoe [Member] | Proved developed reserves [Member] |  |  |
Reserve Quantities [Line Items] |  |  |
Net proved reserves, closing balance | 70,051 | 35,839 | 17,018
MBoe [Member] | Proved undeveloped reserves [Member] |  |  |
Reserve Quantities [Line Items] |  |  |
Net proved reserves, closing balance | 73,294 | 42,876 | 22,762

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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 5 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e61831-109447 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 19 -Paragraph 59F -Subparagraph d -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 10 -Paragraph a -Subparagraph 2 -Article 4 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e61831-109447 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-10.(a)(2)) -URI http://asc.fasb.org/extlink&oid=21918352&loc=d3e511914-122862 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 19 -Paragraph 59F -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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- Definition Production of proved reserves. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 5 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e61831-109447 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 19 -Paragraph 59F -Subparagraph e -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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- Definition Purchase of minerals in place. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 5 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e61831-109447 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 19 -Paragraph 59F -Subparagraph c -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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- Definition Revisions represent changes in previous estimates of proved reserves, either upward or downward, resulting from new information (except for an increase in proved acreage) normally obtained from development drilling and production history or resulting from change in economic factors. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 5 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e61831-109447 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 19 -Paragraph 59F -Subparagraph a -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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