Supplemental Crude Oil and Natural Gas Information - Schedule of Proved Crude Oil and Natural Gas Reserves (Detail)	12 Months Ended
	Dec. 31, 2013 MBoe	Dec. 31, 2012 MBoe	Dec. 31, 2011 MBoe
Changes in Proved Reserves [Roll Forward]
Proved reserves at beginning of period, Total	784,677	508,438	364,712
Revisions of previous estimates, Total	(96,054)	4,149	2,237
Extensions, discoveries and other additions, Total	444,654	233,652	161,981
Production, Total	(49,610)	(35,716)	(22,581)
Sales of minerals in place, Total	0	(7,838)	0
Purchases of minerals in place, Total	458	81,992	2,089
Proved reserves at end of period, Total	1,084,125	784,677	508,438
Crude Oil [Member]
Changes in Proved Reserves [Roll Forward]
Proved reserves at beginning of period	561,163	326,133	224,784
Revisions of previous estimates	(55,783)	33,272	28,607
Extensions, discoveries and other additions	267,009	166,844	87,465
Production	(34,989)	(25,070)	(16,469)
Sales of minerals in place	0	(7,165)	0
Purchases of minerals in place	388	67,149	1,746
Proved reserves at end of period	737,788	561,163	326,133
Natural Gas [Member]
Changes in Proved Reserves [Roll Forward]
Proved reserves at beginning of period	1,341,084	1,093,832	839,568
Revisions of previous estimates	(241,623)	(174,736)	(158,219)
Extensions, discoveries and other additions	1,065,870	400,848	447,098
Production	(87,730)	(63,875)	(36,671)
Sales of minerals in place	0	(4,046)	0
Purchases of minerals in place	419	89,061	2,056
Proved reserves at end of period	2,078,020	1,341,084	1,093,832