Summary of Proved Oil Reserves Based on Year-End Prices for 2008 and Average Prices for 2009 - 2011 (Detail) (Oil Reserves)	12 Months Ended
	Dec. 31, 2011 bbl	Dec. 31, 2010 bbl	Dec. 31, 2009 bbl
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped reserves, Beginning Balance	308,000,000	313,400,000	173,600,000
Synthetic reserves presented as proved under SEC rules			131,600,000
Revisions of previous estimates	21,200,000	22,500,000	5,800,000
Improved recovery	14,200,000	5,800,000	31,000,000
Extensions and discoveries	43,900,000	12,600,000	23,900,000
Production	(37,600,000)	(46,300,000)	(48,200,000)
Sales of properties			(4,300,000)
Proved developed and undeveloped reserves, Ending Balance	349,700,000	308,000,000	313,400,000
Proved developed reserves	238,500,000	248,300,000	270,000,000
Proved undeveloped reserves	111,200,000	59,700,000	43,400,000
Crude Oil
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped reserves, Beginning Balance	178,800,000	183,900,000	173,600,000
Synthetic reserves presented as proved under SEC rules			0
Revisions of previous estimates	16,000,000	18,000,000	3,200,000
Improved recovery	14,200,000	5,800,000	31,000,000
Extensions and discoveries	43,900,000	12,600,000	23,900,000
Production	(32,700,000)	(41,500,000)	(43,500,000)
Sales of properties			(4,300,000)
Proved developed and undeveloped reserves, Ending Balance	220,200,000	178,800,000	183,900,000
Proved developed reserves	118,000,000	129,200,000	150,300,000
Proved undeveloped reserves	102,200,000	49,600,000	33,600,000
Synthetic Oil
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped reserves, Beginning Balance	129,200,000	129,500,000	0
Synthetic reserves presented as proved under SEC rules			131,600,000
Revisions of previous estimates	5,200,000	4,500,000	2,600,000
Improved recovery	0	0	0
Extensions and discoveries	0	0	0
Production	(4,900,000)	(4,800,000)	(4,700,000)
Sales of properties			0
Proved developed and undeveloped reserves, Ending Balance	129,500,000	129,200,000	129,500,000
Proved developed reserves	120,500,000	119,100,000	119,700,000
Proved undeveloped reserves	9,000,000	10,100,000	9,800,000
United States | Crude Oil
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped reserves, Beginning Balance	26,600,000	26,400,000	26,800,000
Synthetic reserves presented as proved under SEC rules			0
Revisions of previous estimates	2,400,000	3,500,000	5,000,000
Improved recovery	0	0	0
Extensions and discoveries	32,600,000	4,100,000	800,000
Production	(6,300,000)	(7,400,000)	(6,200,000)
Sales of properties			0
Proved developed and undeveloped reserves, Ending Balance	55,300,000	26,600,000	26,400,000
Proved developed reserves	20,800,000	15,800,000	18,300,000
Proved undeveloped reserves	34,500,000	10,800,000	8,100,000
Canada | Crude Oil
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped reserves, Beginning Balance	32,800,000	27,800,000	24,300,000
Synthetic reserves presented as proved under SEC rules			0
Revisions of previous estimates	3,100,000	5,400,000	7,200,000
Improved recovery	0	1,000,000	0
Extensions and discoveries	6,700,000	5,000,000	3,300,000
Production	(6,000,000)	(6,400,000)	(7,000,000)
Sales of properties			0
Proved developed and undeveloped reserves, Ending Balance	36,600,000	32,800,000	27,800,000
Proved developed reserves	32,600,000	28,600,000	26,200,000
Proved undeveloped reserves	4,000,000	4,200,000	1,600,000
Canada | Synthetic Oil
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped reserves, Beginning Balance	129,200,000	129,500,000	0
Synthetic reserves presented as proved under SEC rules			131,600,000
Revisions of previous estimates	5,200,000	4,500,000	2,600,000
Improved recovery	0	0	0
Extensions and discoveries	0	0	0
Production	(4,900,000)	(4,800,000)	(4,700,000)
Sales of properties			0
Proved developed and undeveloped reserves, Ending Balance	129,500,000	129,200,000	129,500,000
Proved developed reserves	120,500,000	119,100,000	119,700,000
Proved undeveloped reserves	9,000,000	10,100,000	9,800,000
Malaysia | Crude Oil
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped reserves, Beginning Balance	98,400,000	110,100,000	100,700,000
Synthetic reserves presented as proved under SEC rules			0
Revisions of previous estimates	8,400,000	4,400,000	(4,900,000)
Improved recovery	10,700,000	4,800,000	31,000,000
Extensions and discoveries	4,600,000	3,500,000	11,200,000
Production	(17,700,000)	(24,400,000)	(27,900,000)
Sales of properties			0
Proved developed and undeveloped reserves, Ending Balance	104,400,000	98,400,000	110,100,000
Proved developed reserves	57,200,000	66,500,000	90,000,000
Proved undeveloped reserves	47,200,000	31,900,000	20,100,000
United Kingdom | Crude Oil
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped reserves, Beginning Balance	10,900,000	11,700,000	17,000,000
Synthetic reserves presented as proved under SEC rules			0
Revisions of previous estimates	8,100,000	400,000	(4,100,000)
Improved recovery	3,500,000	0	0
Extensions and discoveries	0	0	0
Production	(900,000)	(1,200,000)	(1,200,000)
Sales of properties			0
Proved developed and undeveloped reserves, Ending Balance	21,600,000	10,900,000	11,700,000
Proved developed reserves	5,100,000	10,900,000	11,700,000
Proved undeveloped reserves	16,500,000	0	0
Republic of the Congo | Crude Oil
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped reserves, Beginning Balance	10,100,000	7,900,000	0
Synthetic reserves presented as proved under SEC rules			0
Revisions of previous estimates	(6,000,000)	4,300,000	0
Improved recovery	0	0	0
Extensions and discoveries	0	0	8,600,000
Production	(1,800,000)	(2,100,000)	(700,000)
Sales of properties			0
Proved developed and undeveloped reserves, Ending Balance	2,300,000	10,100,000	7,900,000
Proved developed reserves	2,300,000	7,400,000	4,100,000
Proved undeveloped reserves	0	2,700,000	3,800,000
Ecuador | Crude Oil
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped reserves, Beginning Balance	0	0	4,800,000
Synthetic reserves presented as proved under SEC rules			0
Revisions of previous estimates	0	0	0
Improved recovery	0	0	0
Extensions and discoveries	0	0	0
Production	0	0	(500,000)
Sales of properties			(4,300,000)
Proved developed and undeveloped reserves, Ending Balance	0	0	0
Proved developed reserves	0	0	0
Proved undeveloped reserves	0	0	0