Long-Term Debt (Narrative) (Details)	Jul. 31, 2019 USD ($)	Apr. 30, 2019 USD ($)	Jul. 31, 2018 USD ($)
Line of Credit Facility [Line Items]
Full borrowing capacity amount	$ 1,100,000,000
Estimated fair value of long-term debt	1,600,000,000	$ 1,600,000,000	$ 1,500,000,000
Revolving credit facility
Line of Credit Facility [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
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 200,000,000.0
Standby letters of credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 50,000,000.0