Principal Sources of Changes in Standardized Measure of Discounted Future Net Cash Flows (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Net changes in prices, production costs and development costs	$ (6,321.2)	$ (370.5)	$ 240.1
Sales and transfers of oil and gas produced, net of production costs	(3,493.3)	(3,273.5)	(2,792.2)
Net change due to extensions and discoveries	4,466.3	3,300.9	1,022.2
Net change due to purchases and sales of proved reserves	347.4	0	48.7
Development costs incurred	3,299.0	1,881.5	1,271.3
Accretion of discount	1,153.5	827.7	698.9
Revisions of previous quantity estimates	728.1	892.5	798.8
Net change in income taxes	9.8	(1,029.4)	(450.2)
Net increase	189.6	2,229.2	837.6
Standardized measure	8,112.8	7,923.2	5,694.0
Beginning of Period
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Standardized measure	7,923.2	5,694.0	4,856.4
End of Period
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Standardized measure	$ 8,112.8	$ 7,923.2	$ 5,694.0