Supplemental Oil and Gas Disclosures - Proved Reserve Data (Detail)	12 Months Ended
	Dec. 31, 2012 MBoe	Dec. 31, 2011 MBoe	Dec. 31, 2010 MBoe	Dec. 31, 2009 MBoe
Reserve Quantities [Line Items]
Proved developed reserves	1,981,945	1,992,327	1,984,991	1,635,565
Proved undeveloped reserves	869,734	997,527	968,274	731,021
Extensions, discoveries and other additions	372
Purchase of minerals in-place	73
Balance	2,989,854	2,953,265	2,366,586
Extensions, discoveries and other additions	372,393	342,078	245,108
Purchases of minerals in-place	73,457	80,420	593,051
Revisions of previous estimates	(298,712)	(59,086)	(11,370)
Production	(284,997)	(273,074)	(240,037)
Sale of properties	(316)	(53,749)	(73)
Balance	2,851,679	2,989,854	2,953,265
Oil Reserves [Member]
Reserve Quantities [Line Items]
Balance	1,369,475	1,308,735	1,067,248
Proved developed reserves	1,009,760	950,091	924,422	762,688
Proved undeveloped reserves	431,217	419,384	384,313	304,560
Extensions, discoveries and other additions	227,401	178,412	129,210
Purchase of minerals in-place	19,019	50,238	248,832
Revisions of previous estimates	(29,678)	(29,487)	(11,339)
Production	(145,096)	(135,446)	(125,143)
Sale of properties	(144)	(2,977)	(73)
Balance	1,440,977	1,369,475	1,308,735	1,067,248
Natural Gas Reserves [Member]
Reserve Quantities [Line Items]
Balance	9,722,273	9,867,182	7,796,031
Proved developed reserves	5,833,105	6,253,418	6,363,417	5,237,265
Proved undeveloped reserves	2,631,101	3,468,855	3,503,765	2,558,766
Extensions, discoveries and other additions	869,949	981,993	695,383
Purchase of minerals in-place	326,627	181,091	2,065,316
Revisions of previous estimates	(1,614,206)	(177,591)	(186)
Production	(839,403)	(825,770)	(689,361)
Sale of properties	(1,034)	(304,632)	(1)
Balance	8,464,206	9,722,273	9,867,182	7,796,031
United States [Member]
Reserve Quantities [Line Items]
Extensions, discoveries and other additions	218
United States [Member] | Oil Reserves [Member]
Reserve Quantities [Line Items]
Balance	794,047	759,015	523,637
Proved developed reserves	629,345	535,741	514,537	373,010
Proved undeveloped reserves	263,957	258,306	244,478	150,627
Extensions, discoveries and other additions	156,621	89,591	72,928
Purchase of minerals in-place	16,172	5,683	195,131
Revisions of previous estimates	(12,033)	(7,191)	7,597
Production	(61,361)	(51,658)	(40,278)
Sale of properties	(144)	(1,393)
Balance	893,302	794,047	759,015	523,637
United States [Member] | Natural Gas Reserves [Member]
Reserve Quantities [Line Items]
Balance	2,976,211	3,272,985	2,437,921
Proved developed reserves	2,353,587	2,215,973	2,284,116	1,785,155
Proved undeveloped reserves	832,320	760,238	988,869	652,766
Extensions, discoveries and other additions	365,863	169,506	102,180
Purchase of minerals in-place	313,885	67,595	951,654
Revisions of previous estimates	(156,840)	(7,716)	47,989
Production	(312,600)	(315,631)	(266,759)
Sale of properties	(612)	(210,528)
Balance	3,185,907	2,976,211	3,272,985	2,437,921
Canada [Member]
Reserve Quantities [Line Items]
Extensions, discoveries and other additions	69
Canada [Member] | Oil Reserves [Member]
Reserve Quantities [Line Items]
Balance	173,041	174,990	146,774
Proved developed reserves	101,691	105,102	113,993	89,222
Proved undeveloped reserves	82,908	67,939	60,997	57,552
Extensions, discoveries and other additions	26,590	22,602	6,816
Purchase of minerals in-place	197	752	42,440
Revisions of previous estimates	(7,146)	(16,343)	(14,592)
Production	(8,083)	(7,376)	(6,375)
Sale of properties		(1,584)	(73)
Balance	184,599	173,041	174,990	146,774
Canada [Member] | Natural Gas Reserves [Member]
Reserve Quantities [Line Items]
Balance	3,547,511	3,491,967	2,305,348
Proved developed reserves	1,734,657	2,108,801	2,181,615	1,436,151
Proved undeveloped reserves	403,227	1,438,710	1,310,352	869,197
Extensions, discoveries and other additions	252,130	505,049	274,755
Purchase of minerals in-place	2,503	8,838	1,064,618
Revisions of previous estimates	(1,443,989)	(133,359)	(8,211)
Production	(219,849)	(230,880)	(144,542)
Sale of properties	(422)	(94,104)	(1)
Balance	2,137,884	3,547,511	3,491,967	2,305,348
Egypt [Member]
Reserve Quantities [Line Items]
Extensions, discoveries and other additions	46
Egypt [Member] | Oil Reserves [Member]
Reserve Quantities [Line Items]
Balance	128,035	127,127	115,593
Proved developed reserves	106,746	105,840	109,657	97,787
Proved undeveloped reserves	17,288	22,195	17,470	17,806
Extensions, discoveries and other additions	36,188	45,039	41,205
Purchase of minerals in-place			11,261
Revisions of previous estimates	(3,678)	(6,185)	(4,723)
Production	(36,511)	(37,946)	(36,209)
Balance	124,034	128,035	127,127	115,593
Egypt [Member] | Natural Gas Reserves [Member]
Reserve Quantities [Line Items]
Balance	982,966	1,076,917	1,159,141
Proved developed reserves	690,436	700,866	748,573	838,000
Proved undeveloped reserves	205,055	282,100	328,344	321,141
Extensions, discoveries and other additions	55,967	77,049	46,692
Purchase of minerals in-place			49,044
Revisions of previous estimates	(13,974)	(37,623)	(41,137)
Production	(129,468)	(133,377)	(136,823)
Balance	895,491	982,966	1,076,917	1,159,141
Australia [Member]
Reserve Quantities [Line Items]
Extensions, discoveries and other additions	36
Australia [Member] | Oil Reserves [Member]
Reserve Quantities [Line Items]
Balance	67,945	66,136	78,441
Proved developed reserves	29,053	35,725	48,072	34,662
Proved undeveloped reserves	34,808	32,220	18,064	43,779
Extensions, discoveries and other additions	6,277	15,762	4,452
Purchase of minerals in-place	276
Revisions of previous estimates	(66)
Production	(10,571)	(13,953)	(16,757)
Balance	63,861	67,945	66,136	78,441
Australia [Member] | Natural Gas Reserves [Member]
Reserve Quantities [Line Items]
Balance	1,569,584	1,488,498	1,361,441
Proved developed reserves	596,052	675,618	682,763	699,963
Proved undeveloped reserves	1,074,018	893,966	805,735	661,478
Extensions, discoveries and other additions	176,969	148,640	199,958
Purchase of minerals in-place	1,745
Revisions of previous estimates	101
Production	(78,329)	(67,554)	(72,901)
Balance	1,670,070	1,569,584	1,488,498	1,361,441
North Sea [Member] | Oil Reserves [Member]
Reserve Quantities [Line Items]
Balance	178,667	154,368	171,714
Proved developed reserves	122,073	145,743	115,705	142,022
Proved undeveloped reserves	28,399	32,924	38,663	29,692
Extensions, discoveries and other additions	592	404	3,383
Purchase of minerals in-place	2,374	43,803
Revisions of previous estimates	(7,257)
Production	(23,904)	(19,908)	(20,729)
Balance	150,472	178,667	154,368	171,714
North Sea [Member] | Natural Gas Reserves [Member]
Reserve Quantities [Line Items]
Balance	108,442	4,144	4,851
Proved developed reserves	93,319	105,028	4,144	4,851
Proved undeveloped reserves	18,985	3,414
Extensions, discoveries and other additions	16,397	475	166
Purchase of minerals in-place	8,494	104,658
Production	(21,029)	(835)	(873)
Balance	112,304	108,442	4,144	4,851
Argentina [Member] | Oil Reserves [Member]
Reserve Quantities [Line Items]
Balance	27,740	27,099	31,089
Proved developed reserves	20,852	21,940	22,458	25,985
Proved undeveloped reserves	3,857	5,800	4,641	5,104
Extensions, discoveries and other additions	1,133	5,014	426
Revisions of previous estimates	502	232	379
Production	(4,666)	(4,605)	(4,795)
Balance	24,709	27,740	27,099	31,089
Argentina [Member] | Natural Gas Reserves [Member]
Reserve Quantities [Line Items]
Balance	537,559	532,671	527,329
Proved developed reserves	365,054	447,132	462,206	473,145
Proved undeveloped reserves	97,496	90,427	70,465	54,184
Extensions, discoveries and other additions	2,623	81,274	71,632
Revisions of previous estimates	496	1,107	1,173
Production	(78,128)	(77,493)	(67,463)
Balance	462,550	537,559	532,671	527,329