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Alternative Energy Tax Credits - Additional Information (Detail) (USD $)	12 Months Ended			3 Months Ended	12 Months Ended		3 Months Ended		12 Months Ended		3 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Mar. 31, 2012 Alternative Fuel Mixture Credits	Dec. 31, 2010 Alternative Fuel Mixture Credits	Dec. 31, 2009 Alternative Fuel Mixture Credits	Dec. 31, 2012 Cellulosic Biofuel Tax Credits	Mar. 31, 2012 Cellulosic Biofuel Tax Credits	Dec. 31, 2010 Cellulosic Biofuel Tax Credits	Dec. 31, 2009 Cellulosic Biofuel Tax Credits	Mar. 31, 2012 Alternative Energy Tax Credits	Dec. 31, 2012 Alternative Energy Tax Credits	Dec. 31, 2010 Alternative Energy Tax Credits	Dec. 31, 2010 Alternative Energy Tax Credits Additional
Alternative Energy Tax Credits [Line Items]
Refundable tax credit per gallon						$ 0.50				$ 1.01
Alternative energy tax credits				95,500,000	(86,300,000)
Provision for income taxes	216,739,000	85,477,000	(52,331,000)				3,400,000	(118,500,000)	135,500,000			81,700,000
Expenses related to alternative energy tax credits														(1,600,000)
Uncertain tax positions			102,000,000
Income (charge) related to alternative energy tax credits											$ (23,000,000)	$ (19,600,000)	$ 47,700,000

The sum of the current income tax expense or benefit and the deferred income tax expense or benefit pertaining to continuing operations.

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