Long-Term Debt (Narrative) (Details)	Jan. 31, 2019 USD ($)	Apr. 30, 2018 USD ($)	Jan. 31, 2018 USD ($)
Line of Credit Facility [Line Items]
Committed line of credit borrowings	$ 385,000,000	$ 0	$ 790,000,000
Full borrowing capacity amount	2,000,000,000
Estimated fair value of long-term debt	1,900,000,000	$ 1,500,000,000	$ 2,400,000,000
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