Stockholders' Equity - Share Repurchase Table (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Oct. 31, 2019	Oct. 31, 2018	Oct. 31, 2019	Oct. 31, 2018
Equity [Abstract]
Cost of repurchases	$ 78.0	$ 71.0	$ 163.4	$ 377.3
Shares repurchased and retired (shares)	0.9	0.6	1.8	3.0
Average cost per share (usd per share)	$ 88.42	$ 118.36	$ 91.15	$ 123.99