Stockholders' Deficit (Narrative) (Details) shares in Millions	3 Months Ended	9 Months Ended
	Oct. 31, 2018 $ / shares shares	Oct. 31, 2018 $ / shares shares
Class of Stock [Line Items]
Common stock repurchased and retired (in shares)	0.8	2.1
Repurchased shares of its common stock on the open market, average repurchase price per share (in usd per share) | $ / shares	$ 131.42	$ 129.86
Common Stock Repurchase Program
Class of Stock [Line Items]
Common stock shares remained available for repurchase under repurchase plans (in shares)	17.6	17.6