Stockholders' Equity - Narrative (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Billions			3 Months Ended		6 Months Ended		12 Months Ended
	Feb. 22, 2018	Feb. 21, 2018	Apr. 30, 2018	Apr. 30, 2017	Apr. 30, 2018	Apr. 30, 2017	Oct. 31, 2017
Equity, Class of Treasury Stock [Line Items]
Cash dividends declared per share, quarterly (in dollars per share)	$ 0.1125	$ 0.075	$ 0.1125	$ 0.065	$ 0.2625	$ 0.195
Share Repurchase program
Equity, Class of Treasury Stock [Line Items]
Common stock retired (in shares)					102.0		1.7
Open share repurchases during period (in shares)					1.4
Repurchases of common stock recorded as a reduction to stockholders' equity					$ 1.7
Share repurchase program additional authorized amount		$ 2.5
Remaining authorized repurchase amount			$ 6.6		$ 6.6