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, 2011	Dec. 31, 2010	Dec. 31, 2009
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	$ 41,809.9	$ 25,795.9	$ 20,274.2
Future development costs	(5,129.0)	(2,464.1)	(1,272.7)
Future production and abandonment costs	(15,725.6)	(10,569.9)	(8,457.2)
Future income taxes	(6,652.4)	(3,915.6)	(3,081.8)
Future net cash flows	14,302.9	8,846.3	7,462.5
10% annual discount for estimated timing of cash flows	(6,379.7)	(3,152.3)	(2,606.1)
Standardized measure of discounted future net cash flows	7,923.2	5,694.0	4,856.4
United States
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	6,105.4	2,472.8	1,908.1
Future development costs	(1,283.5)	(379.7)	(245.7)
Future production and abandonment costs	(1,417.9)	(631.3)	(523.6)
Future income taxes	(807.2)	(335.0)	(264.8)
Future net cash flows	2,596.8	1,126.8	874.0
10% annual discount for estimated timing of cash flows	(912.0)	(254.2)	(174.8)
Standardized measure of discounted future net cash flows	1,684.8	872.6	699.2
Canada
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	18,835.5	13,440.2	9,571.1
Future development costs	(1,929.5)	(1,244.5)	(191.3)
Future production and abandonment costs	(7,199.7)	(6,923.3)	(5,450.6)
Future income taxes	(2,806.8)	(1,509.8)	(952.3)
Future net cash flows	6,899.5	3,762.6	2,976.9
10% annual discount for estimated timing of cash flows	(3,658.7)	(1,818.0)	(1,521.4)
Standardized measure of discounted future net cash flows	3,240.8	1,944.6	1,455.5
Malaysia
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	14,110.9	8,118.9	7,496.1
Future development costs	(1,559.8)	(756.3)	(726.3)
Future production and abandonment costs	(6,161.2)	(2,379.0)	(1,976.1)
Future income taxes	(2,129.7)	(1,659.4)	(1,531.3)
Future net cash flows	4,260.2	3,324.2	3,262.4
10% annual discount for estimated timing of cash flows	(1,507.4)	(922.2)	(838.0)
Standardized measure of discounted future net cash flows	2,752.8	2,402.0	2,424.4
United Kingdom
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	2,509.5	1,005.1	831.2
Future development costs	(356.2)	(14.0)	(9.7)
Future production and abandonment costs	(763.0)	(305.6)	(330.4)
Future income taxes	(869.0)	(329.0)	(250.2)
Future net cash flows	521.3	356.5	240.9
10% annual discount for estimated timing of cash flows	(304.4)	(123.5)	(64.7)
Standardized measure of discounted future net cash flows	216.9	233.0	176.2
Republic of the Congo
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	248.6	758.9	467.7
Future development costs	0	(69.6)	(99.7)
Future production and abandonment costs	(183.8)	(330.7)	(176.5)
Future income taxes	(39.7)	(82.4)	(83.2)
Future net cash flows	25.1	276.2	108.3
10% annual discount for estimated timing of cash flows	2.8	(34.4)	(7.2)
Standardized measure of discounted future net cash flows	$ 27.9	$ 241.8	$ 101.1