Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011		Dec. 31, 2010
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	$ 43,809.2	$ 38,736.5	[1]	$ 25,795.9
Future development costs	(6,786.9)	(5,129.0)		(2,464.1)
Future production and abandonment costs	(14,684.7)	(12,652.2)	[1]	(10,569.9)
Future income taxes	(6,791.1)	(6,652.4)		(3,915.6)
Future net cash flows	15,546.5	14,302.9		8,846.3
10% annual discount for estimated timing of cash flows	(7,433.7)	(6,379.7)		(3,152.3)
Standardized measure of discounted future net cash flows	8,112.8	7,923.2		5,694.0
United States
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	15,547.5	6,105.4		2,472.8
Future development costs	(3,731.6)	(1,283.5)		(379.7)
Future production and abandonment costs	(3,466.6)	(1,417.9)		(631.3)
Future income taxes	(2,527.6)	(807.2)		(335.0)
Future net cash flows	5,821.7	2,596.8		1,126.8
10% annual discount for estimated timing of cash flows	(2,862.1)	(912.0)		(254.2)
Standardized measure of discounted future net cash flows	2,959.6	1,684.8		872.6
Canada
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	15,511.6	18,835.5		13,440.2
Future development costs	(1,815.2)	(1,929.5)		(1,244.5)
Future production and abandonment costs	(7,336.4)	(7,199.7)		(6,923.3)
Future income taxes	(1,714.9)	(2,806.8)		(1,509.8)
Future net cash flows	4,645.1	6,899.5		3,762.6
10% annual discount for estimated timing of cash flows	(2,876.5)	(3,658.7)		(1,818.0)
Standardized measure of discounted future net cash flows	1,768.6	3,240.8		1,944.6
Malaysia
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	10,354.9	11,037.5	[1]	8,118.9
Future development costs	(966.9)	(1,559.8)		(756.3)
Future production and abandonment costs	(3,143.4)	(3,087.8)	[1]	(2,379.0)
Future income taxes	(1,675.9)	(2,129.7)		(1,659.4)
Future net cash flows	4,568.7	4,260.2		3,324.2
10% annual discount for estimated timing of cash flows	(1,322.9)	(1,507.4)		(922.2)
Standardized measure of discounted future net cash flows	3,245.8	2,752.8		2,402.0
United Kingdom
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	2,395.2	2,509.5		1,005.1
Future development costs	(273.2)	(356.2)		(14.0)
Future production and abandonment costs	(738.3)	(763.0)		(305.6)
Future income taxes	(872.7)	(869.0)		(329.0)
Future net cash flows	511.0	521.3		356.5
10% annual discount for estimated timing of cash flows	(372.2)	(304.4)		(123.5)
Standardized measure of discounted future net cash flows	138.8	216.9		233.0
Republic of the Congo
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	0	248.6		758.9
Future development costs	0	0		(69.6)
Future production and abandonment costs	0	(183.8)		(330.7)
Future income taxes	0	(39.7)		(82.4)
Future net cash flows	0	25.1		276.2
10% annual discount for estimated timing of cash flows	0	2.8		(34.4)
Standardized measure of discounted future net cash flows	$ 0	$ 27.9		$ 241.8

[1] Reclassified to conform to current presentation.