Fair Value of Financial Instruments and Long Term Debt - Carrying Value of Long-term Debt (Details) $ in Thousands	12 Months Ended
Apr. 30, 2018 USD ($) installment_payment	Apr. 30, 2017 USD ($) installment_payment
Debt Instrument
Long-term debt	$ 1,307,099	$ 922,777
Less current maturities	15,374	15,421
Long-term debt, net of current maturities	1,291,725	907,356
Capital Lease Obligations
Debt Instrument
Capitalized lease obligations	8,099	8,777
Long-term debt	8,099
Senior Notes
Debt Instrument
Long-term debt	1,299,000
Senior Notes \| 5.72% Senior notes due in 14 installments beginning September 30, 2012 and ending March 30, 2020
Debt Instrument
Long-term debt	$ 30,000	$ 45,000
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	$ 50,000
Interest rate	3.65%	3.65%
Number of payments \| installment_payment	7	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	$ 50,000
Interest rate	3.72%	3.72%
Number of payments \| installment_payment	7	7
Senior Notes \| 3.51% Senior notes (Series E) due June 13, 2025
Debt Instrument
Long-term debt	$ 150,000	$ 0
Interest rate	3.51%
Senior Notes \| 3.77% Senior notes (Series F) due August 22, 2028
Debt Instrument
Long-term debt	$ 250,000	$ 0
Interest rate	3.77%
Measurement Input, Discount Rate [Member] \| Capital Lease Obligations \| Minimum
Debt Instrument
Discount rate	0.0370	0.0370
Measurement Input, Discount Rate [Member] \| Capital Lease Obligations \| Maximum
Debt Instrument
Discount rate	0.0600	0.0600

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.

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- Definition

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

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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

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.

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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.

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- Definition

Value of input used to measure long-term debt.

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