Principal Sources of Change in Standardized Measure of Discounted Future Net Cash Flows (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Inclusion of synthetic oil reserves beginning in 2009	$ 0	[1]	$ 0	[1]	$ 378.9	[1]
Net changes in prices, production costs and development costs	(370.5)		240.1		675.1
Sales and transfers of oil and gas produced, net of production costs	(3,273.5)		(2,792.2)		(2,381.5)
Net change due to extensions and discoveries	3,300.9		1,022.2		1,976.2
Net change due to purchases and sales of proved reserves	0		48.7		(36.7)
Development costs incurred	1,881.5		1,271.3		1,344.1
Accretion of discount	827.7		698.9		422.1
Revisions of previous quantity estimates	892.5		798.8		267.8
Net change in income taxes	(1,029.4)		(450.2)		(454.8)
Net increase	2,229.2		837.6		2,191.2
Standardized measure	7,923.2		5,694.0		4,856.4
Beginning of Period
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Standardized measure	5,694.0		4,856.4		2,665.2
End 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

[1] Prior to 2009, discounted future net cash flows from synthetic oil operations were excluded from this report. With the SEC's change in the definition of proved reserves to include synthetic oil as proved reserves, the Company has included synthetic oil reserves in this table beginning in 2009.