Long-Term Debt (Details)	Jan. 31, 2018 USD ($)	Apr. 30, 2017 USD ($)	Jan. 31, 2017 USD ($)
Debt Instrument [Line Items]
Capital lease obligation	$ 5,878,000	$ 6,610,000	$ 6,820,000
Long-term debt including current portion	2,285,246,000	1,493,998,000	2,593,564,000
Less: Current portion	(1,015,000)	(981,000)	(942,000)
Long-term debt excluding current portion	2,284,231,000	1,493,017,000	2,592,622,000
Outstanding balance under the 2017 CLOC	790,000,000	0	1,100,000,000
Full borrowing capacity amount	2,000,000,000
Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate principal amount	2,000,000,000.0
Available increase in borrowing capacity	$ 500,000,000.0
Maximum quarterly debt-to-EBITDA ratio	3.50
Maximum annual debt-to-EBITDA ratio	4.50
Minimum interest coverage ratio	2.50
Swingline Loans
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 200,000,000.0
Standby Letters of Credit
Debt Instrument [Line Items]
Maximum borrowing capacity	50,000,000.0
Senior Notes \| Senior Notes, 4.125%, due October 2020
Debt Instrument [Line Items]
Senior notes	650,000,000	650,000,000	650,000,000
Senior Notes \| Senior Notes, 5.500%, due November 2022
Debt Instrument [Line Items]
Senior notes	500,000,000	500,000,000	500,000,000
Senior Notes \| Senior Notes, 5.250%, due October 2025
Debt Instrument [Line Items]
Senior notes	350,000,000	350,000,000	350,000,000
Capital Lease Obligations
Debt Instrument [Line Items]
Debt issuance costs and discounts	$ (10,632,000)	$ (12,612,000)	$ (13,256,000)

Debt Instrument, Covenant, Annual Debt To EBITDA Ratio

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Debt Instrument, Covenant, Interest Coverage Ratio

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Debt Instrument, Covenant, Quarterly Debt To EBITDA Ratio

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Line Of Credit Facility, Available Increase In Borrowing Capacity

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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
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-SubTopic 30
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Amount of unused borrowing capacity under the long-term financing arrangement that is available to the entity as of the balance sheet date.

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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 current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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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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Amount of long-term debt and capital lease obligations including current maturities.

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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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