Long-Term Debt and Capital Leases - Components of Long-Term Debt and Capital Leases (Detail) (USD $) In Thousands, unless otherwise specified	Apr. 30, 2014	Apr. 30, 2013
Debt Instrument [Line Items]
Long term debt and capital lease obligations current and non current	$ 508,019	$ 496,205
Less-current maturities	885	1,218
Non current portion of long term debt and capital lease obligation	507,134	494,987
Senior Subordinated Notes Due 2019 Notes [Member]
Debt Instrument [Line Items]
Senior subordinated notes due	323,509	323,265
Senior Secured Revolving Credit Facility, Due 2016 [Member]
Debt Instrument [Line Items]
Senior secured revolving credit facility	133,860	123,200
Revenue Bonds Series, 2005R-1, Due 2025 [Member]
Debt Instrument [Line Items]
Other long term debt	3,600	3,600
Revenue Bonds Series 2005R-2, due 2025 [Member]
Debt Instrument [Line Items]
Other long term debt	21,400	21,400
Long-Term Revenue Bonds Series 2013 Due April 1, 2036 [Member]
Debt Instrument [Line Items]
Other long term debt	16,000	16,000
Revenue Bonds Series 2013 Due April 1, 2029 [Member]
Debt Instrument [Line Items]
Other long term debt	5,500	5,500
Notes Payable [Member]
Debt Instrument [Line Items]
Notes payable	440	1,228
Capital Lease Obligations [Member]
Debt Instrument [Line Items]
Capital leases for facilities and equipment	$ 3,710	$ 2,012

Amount equal to the present value (the principal) at the beginning of the lease term of minimum lease payments during the lease term (excluding that portion of the payments representing executory costs such as insurance, maintenance, and taxes to be paid by the lessor, together with any profit thereon) net of payments or other amounts applied to the principal through the balance sheet date.

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Sum of the carrying values as of the balance sheet date of all debt, including all short-term borrowings, long-term debt, and capital lease obligations.

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Amount borrowed under the credit facility as of the balance sheet date.

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Amount of long-term debt and capital lease obligation due after one year or beyond the normal operating cycle, if longer.

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Including both current and noncurrent portions, carrying amount as of the balance sheet date of other forms of debt not elsewhere specified in the taxonomy with initial maturities beyond one year or beyond the normal operating cycle, if longer.

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Including the current and noncurrent portions, the carrying value of notes payable which were initially due after one year or beyond the normal operating cycle, if longer, and which are not otherwise defined in the taxonomy.

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Including the current and noncurrent portions, carrying value as of the balance sheet date of subordinated debt (with initial maturities beyond one year or beyond the operating cycle if longer). Subordinated debt places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets.

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