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Supplemental Oil and Gas Reserve Information - Additional Information (Detail) | 12 Months Ended | 1 Months Ended | 12 Months Ended
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Dec. 31, 2012 MBbls MBoe | Dec. 31, 2011 MBbls MBoe | Dec. 31, 2010 Property MBbls MBoe | Dec. 31, 2009 | Nov. 30, 2010 Roosevelt County, Montana [Member] MBbls | Dec. 31, 2010 Richland County, Montana [Member] MBbls | Dec. 31, 2012 MBoe [Member] MBbls | Dec. 31, 2011 MBoe [Member] MBbls | Dec. 31, 2010 MBoe [Member] MBbls | Dec. 31, 2012 MBoe [Member] Burke and Mountrail counties [Member] MBbls
Reserve Quantities [Line Items] |  |  |  |  |  |  |  |  |  |
Purchases of estimated net proved reserves in place |  |  |  |  | 715 | 764 | 966 | 111 | 1,479 | 966
Extensions, discoveries and other additions | 73,871 | 43,168 | 24,473 |  |  |  | 73,871 | 43,168 | 24,473 |
Estimated extensions and discoveries of reserves associated with new wells | 16,548 | 12,696 | 8,122 |  |  |  |  |  |  |
Additional proved undeveloped reserves | 57,322 | 30,472 | 16,351 |  |  |  |  |  |  |
Percentage of reserves produced from wells in certain formations | 100.00% | 100.00% | 99.00% |  |  |  |  |  |  |
Percentage of proved undeveloped reserves | 100.00% | 100.00% | 100.00% |  |  |  |  |  |  |
Number of traded interests in non-operated properties |  |  | 3 |  |  |  |  |  |  |
Remaining reserves |  |  |  |  |  |  |  | (142) | (123) |
Net positive (negative) revisions of previous estimates |  |  |  |  |  |  | (1,983) | (288) | 2,552 |
Percentage of Net positive (negative) revisions | (2.50%) | (0.72%) |  |  |  |  |  |  |  |
Percentage of revisions due to increase in oil prices |  |  | 29.00% |  |  |  |  |  |  |
Unweighted arithmetic average first-day-of-the-month prices for oil, price per Bbl | 94.68 | 96.23 | 79.40 | 61.04 |  |  |  |  |  |
Percentage of revisions due to higher working interests in proved wells |  |  | 29.00% |  |  |  |  |  |  |
Remaining reserves |  |  | 42.00% |  |  |  |  |  |  |
Discount rate of future cash flows | 10.00% |  |  |  |  |  |  |  |  |
Unweighted arithmetic average first-day-of-the-month prices for natural gas, price per MMBtu | 2.75 | 4.12 | 4.38 |  |  |  |  |  |  |

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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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