Changes in Estimated Reserves (Detail)
 12 Months Ended
Dec. 31, 2013
Boe
Dec. 31, 2012
Boe
Dec. 31, 2011
Boe
Oil Equivalent
      
Reserve Quantities [Line Items]      
Proved reserves, beginning balance 78,774,000 [1] 64,798,000 [1] 64,669,000 [1]
Purchases of minerals in place 7,818,000 [1] 13,374,000 [1] 4,026,000 [1]
Production (4,467,000) [1] (3,409,000) [1] (2,885,000) [1]
Extensions, discoveries and other additions 5,805,000 [1],[2] 4,924,000 [1],[2] 1,352,000 [1]
Sales of minerals in place (5,906,000) [1] (2,717,000) [1] (293,000) [1]
Revisions of previous estimates (22,630,000) [1],[3] 1,804,000 [1] (2,071,000) [1]
Proved reserves, ending balance 59,394,000 [1] 78,774,000 [1] 64,798,000 [1]
Proved developed reserves 46,842,000 [1] 46,217,000 [1] 36,871,000 [1]
Proved undeveloped reserves 12,552,000 [1] 32,557,000 [1] 27,927,000 [1]
Gas
      
Reserve Quantities [Line Items]      
Proved reserves, beginning balance 47,721,000 35,157,000 39,098,000
Purchases of minerals in place 10,821,000 10,375,000 5,251,000
Production (4,565,000) (3,567,000) (3,415,000)
Extensions, discoveries and other additions 6,128,000 [2] 3,400,000 [2] 890,000
Sales of minerals in place (3,379,000) (735,000) (8,000)
Revisions of previous estimates (14,337,000) [3] 3,091,000 (6,659,000)
Proved reserves, ending balance 42,389,000 47,721,000 35,157,000
Proved developed reserves 29,488,000 25,568,000 17,523,000
Proved undeveloped reserves 12,901,000 22,153,000 17,634,000
Oil
      
Reserve Quantities [Line Items]      
Proved reserves, beginning balance 62,557,000 52,841,000 50,232,000
Purchases of minerals in place 4,592,000 9,463,000 1,811,000
Production (3,499,000) (2,773,000) (2,299,000)
Extensions, discoveries and other additions 4,038,000 [2] 4,115,000 [2] 1,204,000
Sales of minerals in place (5,124,000) (2,391,000) (291,000)
Revisions of previous estimates (15,053,000) [3] 1,302,000 2,184,000
Proved reserves, ending balance 47,511,000 62,557,000 52,841,000
Proved developed reserves 38,791,000 39,288,000 32,347,000
Proved undeveloped reserves 8,720,000 23,268,000 20,494,000
NGL
      
Reserve Quantities [Line Items]      
Proved reserves, beginning balance 8,264,000 6,097,000 7,919,000
Purchases of minerals in place 1,422,000 2,182,000 1,340,000
Production (207,000) (41,000) (17,000)
Extensions, discoveries and other additions 746,000 [2] 243,000 [2]  
Sales of minerals in place (219,000) (203,000)  
Revisions of previous estimates (5,189,000) [3] (14,000) (3,145,000)
Proved reserves, ending balance 4,817,000 8,264,000 6,097,000
Proved developed reserves 3,136,000 2,668,000 1,603,000
Proved undeveloped reserves 1,681,000 5,596,000 4,494,000
[1] In accordance with SEC and Financial Accounting Standards Board ("FASB") requirements, our estimated net proved reserves and standardized measure at December 31, 2013, were determined utilizing prices equal to the respective twelve-month unweighted arithmetic average of first day of the month prices, resulting in an average NYMEX West Texas Intermediate oil price of $96.94 per Bbl for the Aneth Properties and Plains Marketing, L.P. West Texas Intermediate oil price of $93.42 per Bbl for the Permian and Wyoming Properties, and an average Henry Hub spot market gas price of $3.67 per one million British thermal units ("MMBtu"). At December 31, 2012 and 2011, we used an average NYMEX West Texas Intermediate oil price of $94.71 and $96.19 per Bbl, and an average Henry Hub spot market gas price of $2.76 and $4.12 per MMBtu, respectively.
[2] Extensions, discoveries and other additions in 2013 are primarily related to new wells drilled and new proved undeveloped reserve locations in the Permian Basin and in 2012 were related to new proved undeveloped reserve locations in Williston Basin in North Dakota and in Aneth Field in Utah and new wells drilled in the Permian Basin in Texas.
[3] Decreases in proved reserves associated with revisions of previous estimates resulted from management's decision to allocate more of its 2014 capital expenditures toward exploitation drilling of higher expected rates-of-return horizontal wells in the Permian Basin and less capital towards development projects in Aneth Field. Deferral of these projects to 2015 and beyond reduced proved undeveloped reserves by approximately 25 MMBoe, as these projects are no longer expected to be converted to developed reserves within five years after their initial recognition as proved reserves as required under SEC rules, offset by approximately 3 MMBoe converted to proved developed during 2013. The deferred projects (the remaining portion of Aneth Unit Phase 4 CO2 injection, approximately 8 MMBoe; the remaining portion of McElmo Unit DCIIC CO2 injection, approximately 8 MMBoe; and Aneth Field membrane plant, approximately 7 MMBoe) will be again eligible for classification as proved undeveloped reserves, at such time as management adopts a definitive plan to complete the projects.