Stockholders' Equity - Share Repurchase Table (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Jan. 31, 2019	Jan. 31, 2018	Jan. 31, 2017
Equity [Abstract]
Cost of repurchases	$ 421.4	$ 99.2	$ 183.6
Shares repurchased and retired (in shares)	3.5	1.0	2.8
Average cost per share (in usd per share)	$ 121.28	$ 94.86	$ 65.24