Summary of Proved Oil Reserves Based on Average Prices (Detail) (Oil Reserves)	12 Months Ended
	Dec. 31, 2012 bbl	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	349,700,000	308,000,000	313,400,000
Revisions of previous estimates	2,600,000	21,200,000	22,500,000
Improved recovery	7,200,000	14,200,000	5,800,000
Extensions and discoveries	84,000,000	43,900,000	12,600,000
Purchases of properties	12,500,000
Production	(41,200,000)	(37,600,000)	(46,300,000)
Proved developed and undeveloped reserves, Ending Balance	414,800,000	349,700,000	308,000,000
Proved developed reserves	267,700,000	238,500,000	248,300,000	270,000,000
Proved undeveloped reserves	147,100,000	111,200,000	59,700,000	43,400,000
Crude Oil
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped reserves, Beginning Balance	220,200,000	178,800,000	183,900,000
Revisions of previous estimates	7,900,000	16,000,000	18,000,000
Improved recovery	7,200,000	14,200,000	5,800,000
Extensions and discoveries	84,000,000	43,900,000	12,600,000
Purchases of properties	12,500,000
Production	(36,100,000)	(32,700,000)	(41,500,000)
Proved developed and undeveloped reserves, Ending Balance	295,700,000	220,200,000	178,800,000
Proved developed reserves	148,600,000	118,000,000	129,200,000	150,300,000
Proved undeveloped reserves	147,100,000	102,200,000	49,600,000	33,600,000
Synthetic Oil
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped reserves, Beginning Balance	129,500,000	129,200,000	129,500,000
Revisions of previous estimates	(5,300,000)	5,200,000	4,500,000
Improved recovery	0	0	0
Extensions and discoveries	0	0	0
Purchases of properties	0
Production	(5,100,000)	(4,900,000)	(4,800,000)
Proved developed and undeveloped reserves, Ending Balance	119,100,000	129,500,000	129,200,000
Proved developed reserves	119,100,000	120,500,000	119,100,000	119,700,000
Proved undeveloped reserves	0	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	55,300,000	26,600,000	26,400,000
Revisions of previous estimates	13,000,000	2,400,000	3,500,000
Improved recovery	0	0	0
Extensions and discoveries	77,300,000	32,600,000	4,100,000
Purchases of properties	6,500,000
Production	(9,500,000)	(6,300,000)	(7,400,000)
Proved developed and undeveloped reserves, Ending Balance	142,600,000	55,300,000	26,600,000
Proved developed reserves	48,000,000	20,800,000	15,800,000	18,300,000
Proved undeveloped reserves	94,600,000	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	36,600,000	32,800,000	27,800,000
Revisions of previous estimates	(3,400,000)	3,100,000	5,400,000
Improved recovery	0	0	1,000,000
Extensions and discoveries	2,900,000	6,700,000	5,000,000
Purchases of properties	6,000,000
Production	(5,300,000)	(6,000,000)	(6,400,000)
Proved developed and undeveloped reserves, Ending Balance	36,800,000	36,600,000	32,800,000
Proved developed reserves	29,500,000	32,600,000	28,600,000	26,200,000
Proved undeveloped reserves	7,300,000	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	129,500,000	129,200,000	129,500,000
Revisions of previous estimates	(5,300,000)	5,200,000	4,500,000
Improved recovery	0	0	0
Extensions and discoveries	0	0	0
Purchases of properties	0
Production	(5,100,000)	(4,900,000)	(4,800,000)
Proved developed and undeveloped reserves, Ending Balance	119,100,000	129,500,000	129,200,000
Proved developed reserves	119,100,000	120,500,000	119,100,000	119,700,000
Proved undeveloped reserves	0	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	104,400,000	98,400,000	110,100,000
Revisions of previous estimates	(400,000)	8,400,000	4,400,000
Improved recovery	7,200,000	10,700,000	4,800,000
Extensions and discoveries	3,800,000	4,600,000	3,500,000
Purchases of properties	0
Production	(19,300,000)	(17,700,000)	(24,400,000)
Proved developed and undeveloped reserves, Ending Balance	95,700,000	104,400,000	98,400,000
Proved developed reserves	67,000,000	57,200,000	66,500,000	90,000,000
Proved undeveloped reserves	28,700,000	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	21,600,000	10,900,000	11,700,000
Revisions of previous estimates	300,000	8,100,000	400,000
Improved recovery	0	3,500,000	0
Extensions and discoveries	0	0	0
Purchases of properties	0
Production	(1,300,000)	(900,000)	(1,200,000)
Proved developed and undeveloped reserves, Ending Balance	20,600,000	21,600,000	10,900,000
Proved developed reserves	4,100,000	5,100,000	10,900,000	11,700,000
Proved undeveloped reserves	16,500,000	16,500,000	0	0
Republic of the Congo | Crude Oil
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped reserves, Beginning Balance	2,300,000	10,100,000	7,900,000
Revisions of previous estimates	(1,600,000)	(6,000,000)	4,300,000
Improved recovery	0	0	0
Extensions and discoveries	0	0	0
Purchases of properties	0
Production	(700,000)	(1,800,000)	(2,100,000)
Proved developed and undeveloped reserves, Ending Balance	0	2,300,000	10,100,000
Proved developed reserves	0	2,300,000	7,400,000	4,100,000
Proved undeveloped reserves	0	0	2,700,000	3,800,000