Fair Value of Financial Instruments and Long Term Debt - Carrying Value of Long-term Debt (Details) $ in Thousands	12 Months Ended
Apr. 30, 2015 USD ($) installment_payment	Apr. 30, 2014 USD ($) installment_payment
Debt Instrument
Long-term debt	$ 853,643	$ 854,195
Less current maturities	15,398	553
Long-term debt, net of current maturities	838,245	853,642
Capital Lease Obligations
Debt Instrument
Capitalized lease obligations	9,643	$ 10,195
Long-term debt	$ 9,643
Capital Lease Obligations \| Minimum
Debt Instrument
Discount rate	5.22%	5.22%
Capital Lease Obligations \| Maximum
Debt Instrument
Discount rate	7.09%	7.09%
5.72% Senior Notes Due March 30, 2020
Debt Instrument
Long-term debt	$ 75,000	$ 75,000
Interest Rate	5.72%	5.72%
Number of payments \| installment_payment	14	14
5.22% Senior Notes Due August 9, 2020
Debt Instrument
Long-term debt	$ 569,000	$ 569,000
Interest Rate	5.22%	5.22%
3.67% Senior Notes (Series A) Due Jun15, 2028
Debt Instrument
Long-term debt	$ 150,000	$ 150,000
Interest Rate	3.67%	3.67%
Number of payments \| installment_payment	7	7
3.75% Senior Notes (Series B) Due December 18, 2028
Debt Instrument
Long-term debt	$ 50,000	$ 50,000
Interest Rate	3.75%	3.75%
Number of payments \| installment_payment	7	7

Debt Instrument, Periodic Payment, Number of Payments

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- Definition

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.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 840

-SubTopic 30

-Section 50

-Paragraph 1

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 16

-Article 7

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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ 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(a)(1))

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (bbb)

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- Definition

Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.16)

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 20, 22

-Article 5

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- Definition

Amount of long-term debt, after unamortized discount or premium, scheduled to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ 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 210.5-02.19,20)

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Carrying amount of long-term debt, net of unamortized discount or premium, excluding amounts to be repaid within one year or the normal operating cycle, if longer (current maturities). Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

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