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Supplemental Oil and Gas Reserve Information - Additional Information (Detail) | 12 Months Ended
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Dec. 31, 2013 MBoe | Dec. 31, 2012 MBoe | Dec. 31, 2011 MBoe
Reserve Quantities [Line Items] |  |  |
Proved Developed and Undeveloped Reserves, Purchases of Minerals in Place (Energy) | 45,731 | 966 | 111
Proved Developed and Undeveloped Reserves, Extensions, Discoveries, and Additions (Energy) | 46,094 | 73,871 | 43,168
Estimated extensions and discoveries of reserves associated with new wells | 22,190 | 16,548 | 12,696
Additional proved undeveloped reserves | 23,904 | 57,323 | 30,472
Percentage of reserves produced from wells in certain formations | 100.00% | 100.00% | 100.00%
Percentage of proved undeveloped reserves | 100.00% | 100.00% | 100.00%
Net positive (negative) revisions of previous estimates | 5,125 | 1,983 | 288
Percentage of net positive (negative) revisions | 3.60% | 2.50% | 0.70%
Discount rate of future cash flows | 10.00% |  |
Unweighted arithmetic average first-day-of-the-month prices for oil, price per Bbl | 96.96 | 94.68 | 96.23
Unweighted arithmetic average first-day-of-the-month prices for natural gas, price per MMBtu | 3.66 | 2.75 | 4.12
Burke and Mountrail counties [Member] |  |  |
Reserve Quantities [Line Items] |  |  |
Proved Developed and Undeveloped Reserves, Purchases of Minerals in Place (Energy) |  | 966 |

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