Long-Term Debt (Components of Long-Term Debt) (Details) - USD ($) $ in Thousands	Jan. 31, 2019	Apr. 30, 2018	Jan. 31, 2018
Debt Instrument [Line Items]
Committed line of credit borrowings	$ 385,000	$ 0	$ 790,000
Capital lease obligation		5,628	5,878
Debt issuance costs and discounts	(8,011)	(9,993)	(10,632)
Long-term debt including current portion	1,876,989	1,495,635	2,285,246
Less: Current portion		(1,026)	(1,015)
Long-term debt excluding current portion	$ 1,876,989	1,494,609	2,284,231
Senior Notes \| Senior Notes, 4.125%, due October 2020
Debt Instrument [Line Items]
Interest rate	4.125%
Senior notes	$ 650,000	650,000	650,000
Senior Notes \| Senior Notes, 5.500%, due November 2022
Debt Instrument [Line Items]
Interest rate	5.50%
Senior notes	$ 500,000	500,000	500,000
Senior Notes \| Senior Notes, 5.250%, due October 2025
Debt Instrument [Line Items]
Interest rate	5.25%
Senior notes	$ 350,000	$ 350,000	$ 350,000

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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The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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

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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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Including the current and noncurrent portions, carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer). Senior note holders are paid off in full before any payments are made to junior note holders.

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The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

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