Fair Value of Financial Instruments and Long Term Debt - Carrying Value of Long-term Debt (Details) $ in Thousands	12 Months Ended
Apr. 30, 2017 USD ($) installment_payment	Apr. 30, 2016 USD ($) installment_payment
Debt Instrument
Long-term debt	$ 922,777	$ 838,244
Less current maturities	15,421	15,375
Long-term debt, net of current maturities	907,356	822,869
Capital Lease Obligations
Debt Instrument
Capitalized lease obligations	8,777	$ 9,244
Long-term debt	$ 8,777
Capital Lease Obligations \| Minimum
Debt Instrument
Discount rate	3.70%	3.70%
Capital Lease Obligations \| Maximum
Debt Instrument
Discount rate	6.00%	6.00%
Senior Notes
Debt Instrument
Long-term debt	$ 914,000
Senior Notes \| 5.72% Senior notes due in 14 installments beginning September 30, 2012 and ending March 30, 2020
Debt Instrument
Long-term debt	$ 45,000	$ 60,000
Interest rate	5.72%	5.72%
Number of payments \| installment_payment	14	14
Senior Notes \| 5.22% Senior notes due August 9, 2020
Debt Instrument
Long-term debt	$ 569,000	$ 569,000
Interest rate	5.22%	5.22%
Senior Notes \| 3.67% Senior notes (Series A) due in 7 installments beginning June 17, 2022, and ending June 15, 2028
Debt Instrument
Long-term debt	$ 150,000	$ 150,000
Interest rate	3.67%	3.67%
Number of payments \| installment_payment	7	7
Senior Notes \| 3.75% Senior notes (Series B) due in 7 installments beginning December 17, 2022 and ending 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
Senior Notes \| 3.65% Senior notes (Series C) due in 7 installments beginning May 2, 2025 and ending May 2, 2031
Debt Instrument
Long-term debt	$ 50,000	$ 0
Interest rate	3.65%
Number of payments \| installment_payment	7
Senior Notes \| 3.72% Senior notes (Series D) due in 7 installments beginning October 28, 2025 and ending October 28, 2031
Debt Instrument
Long-term debt	$ 50,000	$ 0
Interest rate	3.72%
Number of payments \| installment_payment	7

Debt Instrument, Periodic Payment, Number of Payments

No definition available.

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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
-Subparagraph (b)
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-Name Accounting Standards Codification
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- Definition

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

+ References

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-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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- Definition

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
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-Paragraph 2
-Subparagraph (bbb)
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- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. 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
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- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

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- Definition

Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts 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.

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