Fair Value Measurements and Derivative Instruments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	May 31, 2013 bu
Fair Value, Option, Quantitative Disclosures [Line Items]
Bushels of corn	7,400,000
Bushels of corn subject to firm sales contracts	1,800,000
Bushels of corn available	5,600,000
Iowa Department of Economic Development Loans [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Carrying value of debt	$ 1.3
Fair value of debt	1.2
Commodity Contract [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Deferred loss, net of tax, recorded in other comprehensive income	$ 0.6