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Fair Value of Financial Instruments - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $)
In Thousands, unless otherwise specified
Oct. 31, 2013
Apr. 30, 2013
Assets:    
Restricted assets $ 645 $ 545
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
   
Assets:    
Restricted assets 645 545
Liabilities:    
Interest rate derivatives      
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]
   
Assets:    
Restricted assets      
Liabilities:    
Interest rate derivatives 3,338 4,229
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
   
Assets:    
Restricted assets      
Liabilities:    
Interest rate derivatives