Cost Incurred in Oil and Gas Property Acquisition, Exploration and Development Activities (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Exploration costs	$ 448.0	[1]	$ 562.5	[1]	$ 473.4	[1]
Development costs	3,521.7	[1]	1,954.3	[1]	1,365.8	[1]
Total costs incurred	4,413.7		2,819.6		2,104.0
Asset Retirement Obligation Costs
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Exploration costs	(1.6)		2.3		3.4
Development costs	173.3		49.1		65.9
Total costs incurred	171.7		51.4		69.3
United States
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Exploration costs	174.5	[1]	253.2	[1]	204.1	[1]
Development costs	1,352.7	[1]	263.9	[1]	98.3	[1]
Total costs incurred	1,704.0		750.9		432.2
United States | Asset Retirement Obligation Costs
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Exploration costs	(1.7)		2.0		3.4
Development costs	37.9		15.8		23.7
Total costs incurred	36.2		17.8		27.1
Canada
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Exploration costs	57.0	[1]	76.0	[1]	1.8	[1]
Development costs	664.5	[1]	871.9	[1]	721.0	[1]
Total costs incurred	978.5		966.4		823.4
Canada | Asset Retirement Obligation Costs
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Exploration costs	0.1		0.3		0
Development costs	80.7		20.1		17.1
Total costs incurred	80.8		20.4		17.1
Malaysia
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Exploration costs	68.8	[1]	0.7	[1]	99.6	[1]
Development costs	1,433.7	[1]	705.5	[1]	396.7	[1]
Total costs incurred	1,502.5		706.2		496.3
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	48.6		0.3		8.6
Total costs incurred	48.6		0.3		8.6
United Kingdom
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Exploration costs	(1.0)	[1],[2]	0.5	[1],[2]	6.7	[1],[2]
Development costs	46.6	[1],[2]	30.5	[1],[2]	15.5	[1],[2]
Total costs incurred	45.6	[2]	31.0	[2]	22.2	[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	(11.5)		10.8		10.7
Total costs incurred	(11.5)		10.8		10.7
Republic of the Congo
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Exploration costs	51.1	[1]	0.5	[1]	93.5	[1]
Development costs	22.6	[1]	78.7	[1]	132.3	[1]
Total costs incurred	73.7		102.7		225.8
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	17.6		2.1		5.8
Total costs incurred	17.6		2.1		5.8
Other Regions
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Exploration costs	97.6	[1]	231.6	[1]	67.7	[1]
Development costs	1.6	[1]	3.8	[1]	2.0	[1]
Total costs incurred	109.4		262.4		104.1
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: 2012 Exploration costs $ (1.7 ) 0.1 0.0 0.0 0.0 0.0 (1.6 ) Development costs 37.9 80.7 48.6 (11.5 ) 17.6 0.0 173.3 $ 36.2 80.8 48.6 (11.5 ) 17.6 0.0 171.7 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

[2] The Company has accounted for U.K. operations as discontinued operations due to the impending sale of these operations in early 2013.