Selected Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 4,851	$ 4,513	$ 3,997	$ 3,737	$ 3,603	$ 3,392	$ 3,281	$ 3,006	$ 17,098	$ 13,282	$ 10,540
Gross profit	3,631	3,379	3,030	2,823	2,657	2,503	2,432	2,239	12,863	9,831	7,767
Income (loss) from operations	(36)	65	58	210	137	92	115	191	297	535	454
Net income (loss)	$ (248)	$ (109)	$ 91	$ 392	$ 362	$ 105	$ 299	$ 344	$ 126	$ 1,110	$ 360
Basic net income (loss) per share (in dollars per share)	$ (0.28)	$ (0.12)	$ 0.12	$ 0.51	$ 0.47	$ 0.14	$ 0.40	$ 0.47	$ 0.15	$ 1.48	$ 0.50
Diluted net income (loss) per share (in dollars per share)	$ (0.28)	$ (0.12)	$ 0.11	$ 0.49	$ 0.46	$ 0.13	$ 0.39	$ 0.46	$ 0.15	$ 1.43	$ 0.49