Summary of Proved Natural Gas Reserves Based on Average Prices (Detail) (Natural Gas Reserves)	12 Months Ended
	Dec. 31, 2012 ft3	Dec. 31, 2011 ft3	Dec. 31, 2010 ft3	Dec. 31, 2009 ft3
Proved developed and undeveloped natural gas reserves
Proved developed and undeveloped reserves, Beginning Balance	1,106,100,000,000	883,100,000,000	754,600,000,000
Revisions of previous estimates	20,200,000,000	12,600,000,000	15,200,000,000
Improved recovery	7,200,000,000	13,800,000,000	(1,000,000,000)
Extensions and discoveries	173,500,000,000	363,500,000,000	220,500,000,000
Purchases of properties	9,400,000,000		24,000,000,000
Production	(179,400,000,000)	(166,900,000,000)	(130,200,000,000)
Proved developed and undeveloped reserves, Ending Balance	1,137,000,000,000	1,106,100,000,000	883,100,000,000
Proved developed reserve	706,000,000,000	711,600,000,000	586,000,000,000	401,600,000,000
Proved undeveloped reserves	431,000,000,000	394,500,000,000	297,100,000,000	353,000,000,000
United States
Proved developed and undeveloped natural gas reserves
Proved developed and undeveloped reserves, Beginning Balance	98,400,000,000	90,800,000,000	89,300,000,000
Revisions of previous estimates	16,500,000,000	(6,300,000,000)	6,600,000,000
Improved recovery	0	0	0
Extensions and discoveries	107,200,000,000	31,100,000,000	14,300,000,000
Purchases of properties	7,000,000,000		0
Production	(19,400,000,000)	(17,200,000,000)	(19,400,000,000)
Proved developed and undeveloped reserves, Ending Balance	209,700,000,000	98,400,000,000	90,800,000,000
Proved developed reserve	78,800,000,000	58,200,000,000	67,000,000,000	73,200,000,000
Proved undeveloped reserves	130,900,000,000	40,200,000,000	23,800,000,000	16,100,000,000
Canada
Proved developed and undeveloped natural gas reserves
Proved developed and undeveloped reserves, Beginning Balance	638,900,000,000	326,900,000,000	124,700,000,000
Revisions of previous estimates	(37,200,000,000)	59,400,000,000	15,200,000,000
Improved recovery	0	0	0
Extensions and discoveries	25,800,000,000	321,500,000,000	194,200,000,000
Purchases of properties	2,400,000,000		24,000,000,000
Production	(79,500,000,000)	(68,900,000,000)	(31,200,000,000)
Proved developed and undeveloped reserves, Ending Balance	550,400,000,000	638,900,000,000	326,900,000,000
Proved developed reserve	415,800,000,000	427,100,000,000	210,100,000,000	89,700,000,000
Proved undeveloped reserves	134,600,000,000	211,800,000,000	116,800,000,000	35,000,000,000
Malaysia
Proved developed and undeveloped natural gas reserves
Proved developed and undeveloped reserves, Beginning Balance	347,800,000,000	434,000,000,000	511,800,000,000
Revisions of previous estimates	41,400,000,000	(32,500,000,000)	(11,200,000,000)
Improved recovery	7,200,000,000	14,800,000,000	(1,000,000,000)
Extensions and discoveries	40,500,000,000	10,900,000,000	12,000,000,000
Purchases of properties	0		0
Production	(79,300,000,000)	(79,400,000,000)	(77,600,000,000)
Proved developed and undeveloped reserves, Ending Balance	357,600,000,000	347,800,000,000	434,000,000,000
Proved developed reserve	197,300,000,000	210,500,000,000	277,500,000,000	209,900,000,000
Proved undeveloped reserves	160,300,000,000	137,300,000,000	156,500,000,000	301,900,000,000
United Kingdom
Proved developed and undeveloped natural gas reserves
Proved developed and undeveloped reserves, Beginning Balance	21,000,000,000	31,400,000,000	28,800,000,000
Revisions of previous estimates	(500,000,000)	(8,000,000,000)	4,600,000,000
Improved recovery	0	(1,000,000,000)	0
Extensions and discoveries	0	0	0
Purchases of properties	0		0
Production	(1,200,000,000)	(1,400,000,000)	(2,000,000,000)
Proved developed and undeveloped reserves, Ending Balance	19,300,000,000	21,000,000,000	31,400,000,000
Proved developed reserve	14,100,000,000	15,800,000,000	31,400,000,000	28,800,000,000
Proved undeveloped reserves	5,200,000,000	5,200,000,000	0	0