Stockholders' Equity (Deficit) (Narrative) (Details) - Common Stock Repurchase Program - $ / shares shares in Millions	3 Months Ended		6 Months Ended
	Jul. 31, 2020	Jul. 31, 2019	Jul. 31, 2020	Jul. 31, 2019
Class of Stock [Line Items]
Common stock repurchased and retired (in shares)	0.1	0.3	1.3	0.9
Repurchased shares of its common stock on the open market, average repurchase price per share (in usd per share)	$ 216.84	$ 159.54	$ 155.20	$ 168.11
Common stock shares remained available for repurchase under repurchase plans (in shares)	13.5	16.6	13.5	16.6