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Subsidiary Guarantors - Schedule of Condensed Consolidating Balance Sheet (Parenthetical) (Detail) (USD $)
Apr. 30, 2013
Apr. 30, 2012
Apr. 30, 2011
Apr. 30, 2010
Class A Common Stock [Member]
        
Condensed Financial Statements, Captions [Line Items]        
Common stock, Authorized shares 100,000,000 100,000,000    
Common stock, par value $ 0.01 $ 0.01    
Common stock, issued shares 38,662,000 25,991,000    
Common stock, outstanding shares 38,662,000 25,991,000 25,589,000 24,944,000
Class B Common Stock [Member]
        
Condensed Financial Statements, Captions [Line Items]        
Common stock, Authorized shares 1,000,000 1,000,000    
Common stock, par value $ 0.01 $ 0.01    
Common stock, votes (in votes per share) 10 10    
Common stock, issued shares 988,000 988,000    
Common stock, outstanding shares 988,000 988,000 988,000 988,000