Capitalized Costs Relating to Oil and Gas Producing Activities (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Capitalized Costs Relating to Oil and Gas Producing Activities, by Geographic Area [Line Items]
Unproved oil and gas properties	$ 1,902.3	$ 1,753.6
Proved oil and gas properties	12,787.7	10,684.2
Gross capitalized costs	14,690.0	12,437.8
Accumulated depreciation, depletion and amortization
Unproved oil and gas properties	(458.8)	(346.8)
Proved oil and gas properties	(5,522.2)	(4,209.6)
Net capitalized costs	8,709.0	7,881.4
Oil Reserves
Capitalized Costs Relating to Oil and Gas Producing Activities, by Geographic Area [Line Items]
Unproved oil and gas properties	1,902.3	1,753.6
Proved oil and gas properties	11,557.1	9,512.6
Gross capitalized costs	13,459.4	11,266.2
Accumulated depreciation, depletion and amortization
Unproved oil and gas properties	(458.8)	(346.8)
Proved oil and gas properties	(5,197.9)	(3,929.3)
Net capitalized costs	7,802.7	6,990.1
United States | Oil Reserves
Capitalized Costs Relating to Oil and Gas Producing Activities, by Geographic Area [Line Items]
Unproved oil and gas properties	868.0	564.7
Proved oil and gas properties	2,276.3	1,869.1
Gross capitalized costs	3,144.3	2,433.8
Accumulated depreciation, depletion and amortization
Unproved oil and gas properties	(188.2)	(126.5)
Proved oil and gas properties	(1,255.1)	(1,074.6)
Net capitalized costs	1,701.0	1,232.7
Canada | Oil Reserves
Capitalized Costs Relating to Oil and Gas Producing Activities, by Geographic Area [Line Items]
Unproved oil and gas properties	539.9	729.0
Proved oil and gas properties	3,756.4	2,861.1
Gross capitalized costs	4,296.3	3,590.1
Accumulated depreciation, depletion and amortization
Unproved oil and gas properties	(223.5)	(200.5)
Proved oil and gas properties	(1,493.4)	(1,258.4)
Net capitalized costs	2,579.4	2,131.2
Canada | Synthetic Oil
Capitalized Costs Relating to Oil and Gas Producing Activities, by Geographic Area [Line Items]
Unproved oil and gas properties	0	0
Proved oil and gas properties	1,230.6	1,171.6
Gross capitalized costs	1,230.6	1,171.6
Accumulated depreciation, depletion and amortization
Unproved oil and gas properties	0	0
Proved oil and gas properties	(324.3)	(280.3)
Net capitalized costs	906.3	891.3
Malaysia | Oil Reserves
Capitalized Costs Relating to Oil and Gas Producing Activities, by Geographic Area [Line Items]
Unproved oil and gas properties	372.8	322.4
Proved oil and gas properties	4,252.5	3,596.0
Gross capitalized costs	4,625.3	3,918.4
Accumulated depreciation, depletion and amortization
Unproved oil and gas properties	0	0
Proved oil and gas properties	(1,474.3)	(1,115.3)
Net capitalized costs	3,151.0	2,803.1
United Kingdom | Oil Reserves
Capitalized Costs Relating to Oil and Gas Producing Activities, by Geographic Area [Line Items]
Unproved oil and gas properties	0	0
Proved oil and gas properties	556.6	526.1
Gross capitalized costs	556.6	526.1
Accumulated depreciation, depletion and amortization
Unproved oil and gas properties	0	0
Proved oil and gas properties	(361.9)	(347.9)
Net capitalized costs	194.7	178.2
Republic of the Congo | Oil Reserves
Capitalized Costs Relating to Oil and Gas Producing Activities, by Geographic Area [Line Items]
Unproved oil and gas properties	37.4	57.9
Proved oil and gas properties	715.3	657.0
Gross capitalized costs	752.7	714.9
Accumulated depreciation, depletion and amortization
Unproved oil and gas properties	(6.1)	(6.0)
Proved oil and gas properties	(613.2)	(129.8)
Net capitalized costs	133.4	579.1
Other Regions | Oil Reserves
Capitalized Costs Relating to Oil and Gas Producing Activities, by Geographic Area [Line Items]
Unproved oil and gas properties	84.2	79.6
Proved oil and gas properties	0	3.3
Gross capitalized costs	84.2	82.9
Accumulated depreciation, depletion and amortization
Unproved oil and gas properties	(41.0)	(13.8)
Proved oil and gas properties	0	(3.3)
Net capitalized costs	$ 43.2	$ 65.8