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Long-Term Debt - Components of Long-Term Debt and Capital Leases (Parenthetical) (Detail) (USD $)
In Thousands, unless otherwise specified
6 Months Ended
Oct. 31, 2014
Apr. 30, 2014
Senior Secured Revolving Credit Facility, Due 2016 [Member]
   
Debt Instrument [Line Items]    
Debt instrument, due date Mar. 31, 2016  
Debt instrument stated percentage 3.75%  
Senior Subordinated Notes Due 2019 Notes [Member]
   
Debt Instrument [Line Items]    
Debt instrument, due date Feb. 28, 2019  
Debt instrument stated percentage 7.75%  
Senior second lien notes, unamortized discount $ 1,363 $ 1,491
Revenue Bonds Series 2005R-2, due January 2025 [Member]
   
Debt Instrument [Line Items]    
Debt instrument, due date Jan. 01, 2025  
Debt instrument stated percentage 6.25%  
Long-Term Revenue Bonds Series 2013 Due April 2036 [Member]
   
Debt Instrument [Line Items]    
Debt instrument, due date Apr. 01, 2036  
Debt instrument stated percentage 4.75%  
Revenue Bond Series 2013 Due April 2029 [Member]
   
Debt Instrument [Line Items]    
Debt instrument, due date Apr. 01, 2029  
Debt instrument stated percentage 4.00%  
Revenue Bonds Series, 2005R-1, Due January 2025 [Member]
   
Debt Instrument [Line Items]    
Debt instrument, due date Jan. 01, 2025  
Revenue Bonds Series 2013 Due 2029 [Member]
   
Debt Instrument [Line Items]    
Debt instrument, due date Jan. 01, 2029  
Notes Payable [Member]
   
Debt Instrument [Line Items]    
Debt instrument maturity date Apr. 30, 2017  
Capital Lease Obligations [Member]
   
Debt Instrument [Line Items]    
Debt instrument maturity date Apr. 30, 2023