Derivative Instruments (Additional Information) (Detail) (USD $) In Thousands, unless otherwise specified	0 Months Ended	1 Months Ended	9 Months Ended	0 Months Ended	9 Months Ended	1 Months Ended		1 Months Ended
	Jul. 30, 2012 6 5/8% Senior Notes Due 2016	Jul. 31, 2012 6 5/8% Senior Notes Due 2016	Sep. 30, 2012 6 5/8% Senior Notes Due 2016	Jul. 17, 2012 3.60% Senior Notes Due 2022	Sep. 30, 2012 3.60% Senior Notes Due 2022	Jul. 31, 2012 Refinancing of Debt 3.60% Senior Notes Due 2022	Jul. 17, 2012 Refinancing of Debt 3.60% Senior Notes Due 2022	Jul. 31, 2012 Treasury Lock	Sep. 30, 2012 Commodity Forward Contracts Designated As Fair Value Hedges gal
Derivative [Line Items]
Feedstock forward contracts, in gallons									53,550,000
Senior notes, principal amount refinanced		$ 250,000
Maturity	2016	2016	2016	2022	2022	2022
Senior notes, principal amount issued				250,000			250,000
Payments to the counterparties								$ 1,940