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Supplemental Oil and Gas Reserve Information - Additional Information (Detail)
 12 Months Ended
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