Stockholders' Equity (Deficit) (Narrative) (Details) - Common Stock Repurchase Program - $ / shares shares in Millions	3 Months Ended		9 Months Ended
	Oct. 31, 2020	Oct. 31, 2019	Oct. 31, 2020	Oct. 31, 2019
Class of Stock [Line Items]
Common stock repurchased and retired (in shares)	0.8	0.8	2.1	1.7
Repurchased shares of its common stock on the open market, average repurchase price per share (in usd per share)	$ 231.26	$ 144.49	$ 185.69	$ 156.16
Common stock shares remained available for repurchase under repurchase plans (in shares)	12.6	15.7	12.6	15.7