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Alternative Energy Tax Credits - Additional Information (Detail) (USD $)
 3 Months Ended 3 Months Ended 3 Months Ended 3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Dec. 31, 2010
Mar. 31, 2012
Alternative Fuel Mixture Credits
Dec. 31, 2009
Alternative Fuel Mixture Credits
Mar. 31, 2012
Cellulosic Biofuel Tax Credits
Dec. 31, 2009
Cellulosic Biofuel Tax Credits
Dec. 31, 2010
Amended Two Thousand Nine Federal Return
Mar. 31, 2012
Alternative Energy Tax Credits
Apr. 15, 2013
Alternative Energy Tax Credits
Subsequent Event
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                 (23,000,000)  
Provision for income taxes 33,382,000 141,583,000       118,500,000        
Alternative energy tax credits   95,500,000   95,500,000            
Change in reserve for unrecognized tax benefit, minimum 0                  
Change in reserve for unrecognized tax benefit, maximum 105,000,000                  
Tax credit available to offset future cash tax payment 69,400,000                 36,700,000
Deferred tax benefit from utilization of alternative energy tax credits $ (32,700,000)