Earnings Per Share - Reconciliation of Denominator Used in Calculation of Basic and Diluted Loss Per Share (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Jan. 31, 2020	Oct. 31, 2019	Jul. 31, 2019	Apr. 30, 2019	Jan. 31, 2019	Oct. 31, 2018	Jul. 31, 2018	Apr. 30, 2018	Jan. 31, 2020	Jan. 31, 2019	Jan. 31, 2018
Numerator:
Net income (loss)	$ (248)	$ (109)	$ 91	$ 392	$ 362	$ 105	$ 299	$ 344	$ 126	$ 1,110	$ 360
Denominator:
Weighted-average shares outstanding for basic earnings per share (in shares)									829	751	715
Effect of dilutive securities:
Convertible senior notes which matured in April 2018 (in shares)									0	1	5
Employee stock awards (in shares)									21	21	14
Warrants which settled in June and July 2018 (in shares)									0	2	1
Adjusted weighted-average shares outstanding and assumed conversions for diluted earnings per share (in shares)									850	775	735