Investments - Schedule of Components of Investment Income (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jul. 31, 2019	Jul. 31, 2018	Jul. 31, 2019	Jul. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Interest income	$ 31	$ 12	$ 57	$ 32
Realized gains	1	0	1	1
Realized losses	(1)	0	(1)	(5)
Total investment income	$ 31	$ 12	$ 57	$ 28