Cost Incurred in Oil and Gas Property Acquisition, Exploration and Development Activities (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Property acquisition costs
Unproved	$ 132,500,000		$ 279,300,000		$ 242,800,000
Proved	311,500,000		23,500,000		22,000,000
Total acquisition costs	444,000,000		302,800,000		264,800,000
Exploration costs	448,000,000	[1]	562,500,000	[1]	473,400,000	[1]
Development costs	3,521,700,000	[1]	1,954,300,000	[1]	1,365,800,000	[1]
Total costs incurred	4,413,700,000		2,819,600,000		2,104,000,000
Charged to expense
Dry hole expense	181,924,000		250,954,000		90,100,000
Geophysical and other costs	69,000,000		120,700,000		94,200,000
Total charged to expense	250,100,000		371,700,000		184,300,000
Property additions	4,163,600,000		2,447,900,000		1,919,700,000
United States
Property acquisition costs
Unproved	107,700,000		233,800,000		129,800,000
Proved	69,100,000		0		0
Total acquisition costs	176,800,000		233,800,000		129,800,000
Exploration costs	174,500,000	[1]	253,200,000	[1]	204,100,000	[1]
Development costs	1,352,700,000	[1]	263,900,000	[1]	98,300,000	[1]
Total costs incurred	1,704,000,000		750,900,000		432,200,000
Charged to expense
Dry hole expense	32,300,000		600,000		(1,400,000)
Geophysical and other costs	19,600,000		35,900,000		37,100,000
Total charged to expense	51,900,000		36,500,000		35,700,000
Property additions	1,652,100,000		714,400,000		396,500,000
Canada
Property acquisition costs
Unproved	14,600,000		18,500,000		78,600,000
Proved	242,400,000		0		22,000,000
Total acquisition costs	257,000,000		18,500,000		100,600,000
Exploration costs	57,000,000	[1]	76,000,000	[1]	1,800,000	[1]
Development costs	664,500,000	[1]	871,900,000	[1]	721,000,000	[1]
Total costs incurred	978,500,000		966,400,000		823,400,000
Charged to expense
Dry hole expense	8,000,000		50,600,000		0
Geophysical and other costs	2,500,000		10,200,000		1,900,000
Total charged to expense	10,500,000		60,800,000		1,900,000
Property additions	968,000,000		905,600,000		821,500,000
Malaysia
Property acquisition costs
Unproved	0		0		0
Proved	0		0		0
Total acquisition costs	0		0		0
Exploration costs	68,800,000	[1]	700,000	[1]	99,600,000	[1]
Development costs	1,433,700,000	[1]	705,500,000	[1]	396,700,000	[1]
Total costs incurred	1,502,500,000		706,200,000		496,300,000
Charged to expense
Dry hole expense	26,100,000		100,000		14,300,000
Geophysical and other costs	1,100,000		11,000,000		5,500,000
Total charged to expense	27,200,000		11,100,000		19,800,000
Property additions	1,475,300,000		695,100,000		476,500,000
United Kingdom
Property acquisition costs
Unproved	0	[2]	0	[2]	0	[2]
Proved	0	[2]	0	[2]	0	[2]
Total acquisition costs	0	[2]	0	[2]	0	[2]
Exploration costs	(1,000,000)	[1],[2]	500,000	[1],[2]	6,700,000	[1],[2]
Development costs	46,600,000	[1],[2]	30,500,000	[1],[2]	15,500,000	[1],[2]
Total costs incurred	45,600,000	[2]	31,000,000	[2]	22,200,000	[2]
Charged to expense
Dry hole expense	(800,000)	[2]	0	[2]	15,200,000	[2]
Geophysical and other costs	(200,000)	[2]	500,000	[2]	1,000,000	[2]
Total charged to expense	(1,000,000)	[2]	500,000	[2]	16,200,000	[2]
Property additions	46,600,000	[2]	30,500,000	[2]	6,000,000	[2]
Republic of the Congo
Property acquisition costs
Unproved	0		0		0
Proved	0		23,500,000		0
Total acquisition costs	0		23,500,000		0
Exploration costs	51,100,000	[1]	500,000	[1]	93,500,000	[1]
Development costs	22,600,000	[1]	78,700,000	[1]	132,300,000	[1]
Total costs incurred	73,700,000		102,700,000		225,800,000
Charged to expense
Dry hole expense	76,200,000		18,100,000		35,500,000
Geophysical and other costs	600,000		2,900,000		20,900,000
Total charged to expense	76,800,000		21,000,000		56,400,000
Property additions	(3,100,000)		81,700,000		169,400,000
Other Regions
Property acquisition costs
Unproved	10,200,000		27,000,000		34,400,000
Proved	0		0		0
Total acquisition costs	10,200,000		27,000,000		34,400,000
Exploration costs	97,600,000	[1]	231,600,000	[1]	67,700,000	[1]
Development costs	1,600,000	[1]	3,800,000	[1]	2,000,000	[1]
Total costs incurred	109,400,000		262,400,000		104,100,000
Charged to expense
Dry hole expense	39,300,000		181,600,000		26,500,000
Geophysical and other costs	45,400,000		60,200,000		27,800,000
Total charged to expense	84,700,000		241,800,000		54,300,000
Property additions	$ 24,700,000		$ 20,600,000		$ 49,800,000

[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.