Stockholders' Equity - Share Repurchase Program (Details) - USD ($) $ / shares in Units, shares in Millions	3 Months Ended		6 Months Ended
	Jul. 31, 2020	Jul. 31, 2019	Jul. 31, 2020	Jul. 31, 2019	May 31, 2018
Class of Stock [Line Items]
Cost of repurchases	$ 0	$ 60,000,000.0	$ 0	$ 85,400,000
Shares repurchased and retired (shares)	0.0	0.6	0.0	0.9
Average cost per share (usd per share)	$ 0	$ 93.82	$ 0	$ 93.80
2018 Share Repurchase Program
Class of Stock [Line Items]
Repurchase program authorized amount					$ 1,000,000,000.0
Repurchase program authorized amount, remaining	$ 471,600,000		$ 471,600,000