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Investments and Restricted Deposits (Detail) (USD $)
 6 Months Ended 6 Months Ended
Jul. 31, 2012
Jul. 31, 2011
Jan. 31, 2012
Jul. 31, 2012
Patriot And Big River [Member]
Jan. 31, 2012
Patriot And Big River [Member]
Jul. 31, 2012
NuGen Energy [Member]
Jan. 31, 2012
NuGen Energy [Member]
Nov. 02, 2011
NuGen Energy [Member]
Jul. 01, 2010
NuGen Energy [Member]
Restricted Investments $ 383,000   $ 743,000 $ 408,100,000 $ 326,200,000        
300,000   620,000            
Business Combination, Step Acquisition, Equity Interest in Acquiree, Percentage             98.00% 50.00% 48.00%
Business Acquisition, Description of Acquired Entity           NuGen which operates an ethanol producing facility in Marion, South Dakota with an annual nameplate capacity of 100 million gallons. The Company accounted for this investment using the equity method of accounting.      
Business Acquisition, Percentage of Voting Interests Acquired           99.00%   100.00%  
Retained Earnings, Undistributed Earnings from Equity Method Investees 20,700,000   22,800,000            
Proceeds from Equity Method Investment, Dividends or Distributions 2,005,000 2,316,000              
Equity Method Investment, Additional Information The restricted net assets of Patriot and Big River combined at July 31, 2012 and January 31, 2012 are approximately $408.1 million and $326.2 million, respectively. The Company's proportionate share of restricted net assets of Patriot and Big River combined at July 31, 2012 and January 31, 2012 are approximately $55.8 million and $44.2 million, respectively.                
Proportionate Share Of Restricted Net Assets       $ 55,800,000 $ 44,200,000