Long-Term Debt and Capital Leases - Components of Long-Term Debt and Capital Leases (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Apr. 30, 2014	Apr. 30, 2013
Debt Instrument [Line Items]
Credit facility	$ 535,711us-gaap_DebtAndCapitalLeaseObligations	$ 508,019us-gaap_DebtAndCapitalLeaseObligations	$ 496,205us-gaap_DebtAndCapitalLeaseObligations
Less-current maturities of long-term debt	1,656	885	1,218
Non current portion of long term debt and capital lease obligation	534,055	507,134	494,987
Senior Secured Revolving Credit Facility, Due 2016 [Member]
Debt Instrument [Line Items]
Credit facility	131,300	133,860	123,200
Revenue Bonds Series 2014 Due December 2044 [Member]
Debt Instrument [Line Items]
Credit facility	25,000
Revenue Bonds Series 2005R-2, due January 2025 [Member]
Debt Instrument [Line Items]
Credit facility	21,400	21,400	21,400
Long-Term Revenue Bonds Series 2013 Due April 2036 [Member]
Debt Instrument [Line Items]
Credit facility	16,000	16,000	16,000
Revenue Bonds Series 2013 Due April 2029 [Member]
Debt Instrument [Line Items]
Credit facility	11,000
Revenue Bonds Series, 2005R-1, Due January 2025 [Member]
Debt Instrument [Line Items]
Credit facility	3,600	3,600	3,600
Revenue Bonds Series 2013 Due April 2029 [Member]
Debt Instrument [Line Items]
Credit facility		5,500	5,500
Capital Lease Obligations [Member]
Debt Instrument [Line Items]
Credit facility	3,295	3,710	2,012
Notes Payable [Member]
Debt Instrument [Line Items]
Credit facility	435	440	1,228
Senior Subordinated Notes Due 2019 Notes [Member]
Debt Instrument [Line Items]
Credit facility	$ 323,681	$ 323,509	$ 323,265

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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.16)

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.13,16)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 6

-Section H

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Amount of long-term debt and capital leases due within one year or the normal operating cycle, if longer.

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