Costs Incurred in Oil and Gas Property Acquisition, Exploration and Development Activities (Detail) (USD $)	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Property acquisition costs
Unproved	$ 279,300,000		$ 242,800,000		$ 118,100,000
Proved	23,500,000		22,000,000		0
Total acquisition costs	302,800,000		264,800,000		118,100,000
Exploration costs	562,500,000	[1]	473,400,000	[1]	312,600,000	[1]
Development costs	1,954,300,000	[1]	1,365,800,000	[1]	1,419,100,000	[1]
Total costs incurred	2,819,600,000		2,104,000,000		1,849,800,000
Charged to expense
Dry hole expense	250,954,000		90,125,000	[2]	125,244,000	[2]
Geophysical and other costs	120,700,000		94,200,000		56,700,000
Total charged to expense	371,700,000	[3]	184,300,000	[3]	182,000,000	[3]
Property additions	2,447,900,000		1,919,700,000		1,667,800,000
United States
Property acquisition costs
Unproved	233,800,000		129,800,000		82,400,000
Proved	0		0		0
Total acquisition costs	233,800,000		129,800,000		82,400,000
Exploration costs	253,200,000	[1]	204,100,000	[1]	89,700,000	[1]
Development costs	263,900,000	[1]	98,300,000	[1]	197,200,000	[1]
Total costs incurred	750,900,000		432,200,000		369,300,000
Charged to expense
Dry hole expense	600,000		(1,400,000)		11,300,000
Geophysical and other costs	35,900,000		37,100,000		16,200,000
Total charged to expense	36,500,000		35,700,000		27,500,000
Property additions	714,400,000		396,500,000		341,800,000
Canada
Property acquisition costs
Unproved	18,500,000		78,600,000		31,000,000
Proved	0		22,000,000		0
Total acquisition costs	18,500,000		100,600,000		31,000,000
Exploration costs	76,000,000	[1]	1,800,000	[1]	9,900,000	[1]
Development costs	871,900,000	[1]	721,000,000	[1]	321,400,000	[1]
Total costs incurred	966,400,000		823,400,000		362,300,000
Charged to expense
Dry hole expense	50,600,000		0		0
Geophysical and other costs	10,200,000		1,900,000		10,000,000
Total charged to expense	60,800,000		1,900,000		10,000,000
Property additions	905,600,000		821,500,000		352,300,000
Malaysia
Property acquisition costs
Unproved	0		0		0
Proved	0		0		0
Total acquisition costs	0		0		0
Exploration costs	700,000	[1]	99,600,000	[1]	114,400,000	[1]
Development costs	705,500,000	[1]	396,700,000	[1]	695,900,000	[1]
Total costs incurred	706,200,000		496,300,000		810,300,000
Charged to expense
Dry hole expense	100,000		14,300,000		55,000,000
Geophysical and other costs	11,000,000		5,500,000		800,000
Total charged to expense	11,100,000		19,800,000		55,800,000
Property additions	695,100,000		476,500,000		754,500,000
United Kingdom
Property acquisition costs
Unproved	0		0		0
Proved	0		0		0
Total acquisition costs	0		0		0
Exploration costs	500,000	[1]	6,700,000	[1]	100,000	[1]
Development costs	30,500,000	[1]	15,500,000	[1]	15,100,000	[1]
Total costs incurred	31,000,000		22,200,000		15,200,000
Charged to expense
Dry hole expense	0		15,200,000		0
Geophysical and other costs	500,000		1,000,000		200,000
Total charged to expense	500,000		16,200,000		200,000
Property additions	30,500,000		6,000,000		15,000,000
Republic of the Congo
Property acquisition costs
Unproved	0		0		0
Proved	23,500,000		0		0
Total acquisition costs	23,500,000		0		0
Exploration costs	500,000	[1]	93,500,000	[1]	19,100,000	[1]
Development costs	78,700,000	[1]	132,300,000	[1]	187,500,000	[1]
Total costs incurred	102,700,000		225,800,000		206,600,000
Charged to expense
Dry hole expense	18,100,000		35,500,000		13,900,000
Geophysical and other costs	2,900,000		20,900,000		(3,100,000)
Total charged to expense	21,000,000		56,400,000		10,800,000
Property additions	81,700,000		169,400,000		195,800,000
Other Regions
Property acquisition costs
Unproved	27,000,000		34,400,000		4,700,000
Proved	0		0		0
Total acquisition costs	27,000,000		34,400,000		4,700,000
Exploration costs	231,600,000	[1]	67,700,000	[1]	79,400,000	[1]
Development costs	3,800,000	[1]	2,000,000	[1]	2,000,000	[1]
Total costs incurred	262,400,000		104,100,000		86,100,000
Charged to expense
Dry hole expense	181,600,000		26,500,000		45,100,000
Geophysical and other costs	60,200,000		27,800,000		32,600,000
Total charged to expense	241,800,000		54,300,000		77,700,000
Property additions	$ 20,600,000		$ 49,800,000		$ 8,400,000

[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

[2] Reclassified to conform to current presentation.

[3] Results exclude corporate overhead, interest and discontinued operations.