Postretirement Benefits - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Apr. 30, 2020	Apr. 30, 2019	Apr. 30, 2020	Apr. 30, 2019	Oct. 31, 2020	Oct. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)				$ 11
Defined Contribution Plan, Administrative Expense				142
Liabilities	$ 10,296		$ 10,296			$ 10,640
Income Tax Expense (Benefit)	7	$ (1)	2	$ (20)
Liability, Defined Benefit Plan, Noncurrent	2,032		$ 2,032			$ 2,103
Defined Contribution Plan, Cost	$ 11
Scenario, Forecast [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions					$ 162