Costs Incurred in Oil and Gas Property Acquisition, Exploration and Development Activities (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Exploration costs	$ 562.5	[1]	$ 473.4	[1]	$ 312.6	[1]
Development costs	1,954.3	[1]	1,365.8	[1]	1,419.1	[1]
Total costs incurred	2,819.6		2,104.0		1,849.8
Asset Retirement Obligation Costs
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Exploration costs	2.3		3.4		5.0
Development costs	49.1		65.9		36.4
Total costs incurred	51.4		69.3		41.4
United States
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Exploration costs	253.2	[1]	204.1	[1]	89.7	[1]
Development costs	263.9	[1]	98.3	[1]	197.2	[1]
Total costs incurred	750.9		432.2		369.3
United States | Asset Retirement Obligation Costs
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Exploration costs	2.0		3.4		5.0
Development costs	15.8		23.7		0
Total costs incurred	17.8		27.1		5.0
Canada
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Exploration costs	76.0	[1]	1.8	[1]	9.9	[1]
Development costs	871.9	[1]	721.0	[1]	321.4	[1]
Total costs incurred	966.4		823.4		362.3
Canada | Asset Retirement Obligation Costs
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Exploration costs	0.3		0		0
Development costs	20.1		17.1		22.2
Total costs incurred	20.4		17.1		22.2
Malaysia
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Exploration costs	0.7	[1]	99.6	[1]	114.4	[1]
Development costs	705.5	[1]	396.7	[1]	695.9	[1]
Total costs incurred	706.2		496.3		810.3
Malaysia | Asset Retirement Obligation Costs
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Exploration costs	0		0		0
Development costs	0.3		8.6		15.5
Total costs incurred	0.3		8.6		15.5
United Kingdom
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Exploration costs	0.5	[1]	6.7	[1]	0.1	[1]
Development costs	30.5	[1]	15.5	[1]	15.1	[1]
Total costs incurred	31.0		22.2		15.2
United Kingdom | Asset Retirement Obligation Costs
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Exploration costs	0		0		0
Development costs	10.8		10.7		(2.2)
Total costs incurred	10.8		10.7		(2.2)
Republic of the Congo
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Exploration costs	0.5	[1]	93.5	[1]	19.1	[1]
Development costs	78.7	[1]	132.3	[1]	187.5	[1]
Total costs incurred	102.7		225.8		206.6
Republic of the Congo | Asset Retirement Obligation Costs
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Exploration costs	0		0		0
Development costs	2.1		5.8		0.9
Total costs incurred	2.1		5.8		0.9
Other Regions
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Exploration costs	231.6	[1]	67.7	[1]	79.4	[1]
Development costs	3.8	[1]	2.0	[1]	2.0	[1]
Total costs incurred	262.4		104.1		86.1
Other Regions | Asset Retirement Obligation Costs
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Exploration costs	0		0		0
Development costs	0		0		0
Total costs incurred	$ 0		$ 0		$ 0

[1] Includes non-cash asset retirement costs as follows: 2011 Exploration costs $ 2.0 0.3 0.0 0.0 0.0 0.0 2.3 Development costs 15.8 20.1 0.3 10.8 2.1 0.0 49.1 $ 17.8 20.4 0.3 10.8 2.1 0.0 51.4 2010 Exploration costs $ 3.4 0.0 0.0 0.0 0.0 0.0 3.4 Development costs 23.7 17.1 8.6 10.7 5.8 0.0 65.9 $ 27.1 17.1 8.6 10.7 5.8 0.0 69.3 2009 Exploration costs $ 5.0 0.0 0.0 0.0 0.0 0.0 5.0 Development costs 0.0 22.2 15.5 (2.2 ) 0.9 0.0 36.4 $ 5.0 22.2 15.5 (2.2 ) 0.9 0.0 41.4