Fair Value of Financial Instruments and Long Term Debt - Carrying Value of Long-term Debt (Details) $ in Thousands	12 Months Ended
Apr. 30, 2020 USD ($) installment_payment	Apr. 30, 2019 USD ($) installment_payment
Debt Instrument
Finance lease liabilities (Note 7)	$ 16,746
Finance lease liabilities (Note 7)		$ 16,480
Long-term debt	1,269,000
Total	1,285,746	1,300,480
Less current maturities (2)	571,244	17,205
Long-term debt and finance lease obligations, net of current maturities	714,502	1,283,275
Debt issuance costs	$ 964	$ 1,171
Senior Notes \| 5.72% Senior notes due in 14 installments beginning September 30, 2012 and ending March 30, 2020
Debt Instrument
Interest rate	5.72%	5.72%
Number of payments \| installment_payment	14	14
Long-term debt	$ 0	$ 15,000
Senior Notes \| 5.22% Senior notes due August 9, 2020 (1)
Debt Instrument
Interest rate	5.22%	5.22%
Long-term debt	$ 569,000	$ 569,000
Senior Notes \| 3.67% Senior notes (Series A) due in 7 installments beginning June 17, 2022, and ending June 15, 2028
Debt Instrument
Interest rate	3.67%	3.67%
Number of payments \| installment_payment	7	7
Long-term debt	$ 150,000	$ 150,000
Senior Notes \| 3.75% Senior notes (Series B) due in 7 installments beginning December 17, 2022 and ending December 18, 2028
Debt Instrument
Interest rate	3.75%	3.75%
Number of payments \| installment_payment	7	7
Long-term debt	$ 50,000	$ 50,000
Senior Notes \| 3.65% Senior notes (Series C) due in 7 installments beginning May 2, 2025 and ending May 2, 2031
Debt Instrument
Interest rate	3.65%	3.65%
Number of payments \| installment_payment	7	7
Long-term debt	$ 50,000	$ 50,000
Senior Notes \| 3.72% Senior notes (Series D) due in 7 installments beginning October 28, 2025 and ending October 28, 2031
Debt Instrument
Interest rate	3.72%	3.72%
Number of payments \| installment_payment	7	7
Long-term debt	$ 50,000	$ 50,000
Senior Notes \| 3.51% Senior notes (Series E) due June 13, 2025
Debt Instrument
Interest rate	3.51%
Long-term debt	$ 150,000	150,000
Senior Notes \| 3.77% Senior notes (Series F) due August 22, 2028
Debt Instrument
Interest rate	3.77%
Long-term debt	$ 250,000	$ 250,000

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

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Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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- Definition

Present value of lessee's discounted obligation for lease payments from finance lease.

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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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Amount of long-term debt and lease obligation, classified as noncurrent.

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Amount of long-term debt and lease obligation, classified as current.

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Amount of long-term debt and lease obligation, including portion classified as current.

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