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Supplemental Oil and Gas Reserve Information - Net Proved, Proved Developed and Proved Undeveloped Reserves (Detail)
 12 Months Ended
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