Stockholders' Equity - Share Repurchase Table (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended	6 Months Ended
Jul. 31, 2019	Jul. 31, 2018	Jul. 31, 2019	Jul. 31, 2018
Equity [Abstract]
Cost of repurchases	$ 60.0	$ 265.9	$ 85.4	$ 306.3
Shares repurchased and retired (shares)	0.6	2.0	0.9	2.4
Average cost per share (usd per share)	$ 93.82	$ 130.54	$ 93.80	$ 125.37

Number of shares that have been repurchased and retired during the period.

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