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Accounts Receivable - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 12 Months Ended
Apr. 30, 2014
Apr. 30, 2013
Jan. 31, 2010
Jan. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Allowance for doubtful accounts $ 5.5     $ 5.5
The amount of the net proceeds remaining from public auction of the partially completed project     5.5  
Provision for accounts receivable loss 0 0    
Minimum [Member]
        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Length of retention period of amount given to project owners 7 months      
Maximum [Member]
        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Length of retention period of amount given to project owners 26 months      
Construction Contracts Retainage [Member]
        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Contract payment amounts retained by project owners $ 27.5     $ 22.7