Supplemental Crude Oil and Natural Gas Information - Schedule of Proved Crude Oil and Natural Gas Reserves (Detail)	12 Months Ended
	Dec. 31, 2012 MBoe	Dec. 31, 2011 MBoe	Dec. 31, 2010 MBoe
Reserve Quantities [Line Items]
Proved reserves at beginning of period, Total	508,438	364,712	257,293
Revisions of previous estimates, Total	4,149	2,237	27,629
Extensions, discoveries and other additions, Total	233,652	161,981	95,233
Production, Total	(35,716)	(22,581)	(15,811)
Sales of minerals in place, Total	(7,838)
Purchases of minerals in place, Total	81,992	2,089	368
Proved reserves at end of period, Total	784,677	508,438	364,712
Crude Oil [Member]
Reserve Quantities [Line Items]
Proved reserves at beginning of period	326,133	224,784	173,280
Revisions of previous estimates	33,272	28,607	14,414
Extensions, discoveries and other additions	166,844	87,465	48,542
Production	(25,070)	(16,469)	(11,820)
Sales of minerals in place	(7,165)
Purchases of minerals in place	67,149	1,746	368
Proved reserves at end of period	561,163	326,133	224,784
Natural Gas [Member]
Reserve Quantities [Line Items]
Proved reserves at beginning of period	1,093,832	839,568	504,080
Revisions of previous estimates	(174,736)	(158,219)	79,285
Extensions, discoveries and other additions	400,848	447,098	280,146
Production	(63,875)	(36,671)	(23,943)
Sales of minerals in place	(4,046)
Purchases of minerals in place	89,061	2,056
Proved reserves at end of period	1,341,084	1,093,832	839,568