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Alternative Energy Tax Credits - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended			9 Months Ended		3 Months Ended	9 Months Ended		3 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2010	Sep. 30, 2012 Alternative Fuel Mixture Credits	Dec. 31, 2009 Alternative Fuel Mixture Credits	Dec. 31, 2010 Cellulosic Biofuel Tax Credits	Sep. 30, 2012 Cellulosic Biofuel Tax Credits	Dec. 31, 2009 Cellulosic Biofuel Tax Credits	Dec. 31, 2010 Amended Two Thousand Nine Federal Return
Alternative Energy Tax Credits [Line Items]
Refundable tax credit per gallon							$ 0.50			$ 1.01
Cellulosic biofuel producer credits											107,000,000
Uncertain tax positions					102,000,000
Alternative energy tax credits						95,500,000		5,000,000	23,000,000
Provision for income taxes	21,691,000	22,676,000	188,722,000	66,429,000					118,500,000
Tax credits used to offset future cash tax payment	$ 78,700,000		$ 78,700,000

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

The amount of the tax credit carryforward, before tax effects, available to reduce future taxable income under enacted tax laws.