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Alternative Energy Tax Credits - Additional Information (Detail) (USD $)
 3 Months Ended 9 Months Ended 3 Months Ended 3 Months Ended 3 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2013
Sep. 30, 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
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 45,930,000 21,691,000 112,885,000 188,722,000       118,500,000      
Alternative energy tax credits       95,500,000   95,500,000          
Change in reserve for unrecognized tax benefit, minimum 0   0                
Change in reserve for unrecognized tax benefit, maximum 105,000,000   105,000,000                
Tax credits used to offset future cash tax payment $ 0   $ 0