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Subsidiary Guarantors (Schedule of Condensed Consolidating Balance Sheet) (Parenthetical) (Detail) (USD $)
Jan. 31, 2013
Apr. 30, 2012
Jan. 31, 2012
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 38,658,000 25,991,000  
Common stock, outstanding shares 38,658,000 25,991,000 25,987,000
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