Alternative Energy Tax Credits - Additional Information (Detail) (USD $)	3 Months Ended		6 Months Ended			6 Months Ended		3 Months Ended	6 Months Ended		3 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2010	Jun. 30, 2012 Alternative Fuel Mixture Credits	Dec. 31, 2009 Alternative Fuel Mixture Credits	Dec. 31, 2010 Cellulosic Biofuel Tax Credits	Jun. 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	25,448,000	22,170,000	167,031,000	43,753,000					118,500,000
Tax credits used to offset future cash tax payment	$ 100,500,000		$ 100,500,000