Net Income Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended				6 Months Ended
	Jul. 31, 2020	Apr. 30, 2020	Jul. 31, 2019	Apr. 30, 2019	Jul. 31, 2020	Jul. 31, 2019
Numerator:
Net income	$ 98.2	$ 66.5	$ 40.2	$ (24.2)	$ 164.7	$ 16.0
Denominator:
Denominator for basic net loss per share—weighted average shares (shares)	219,200,000		219,600,000		219,200,000	219,600,000
Effect of dilutive securities (shares)	3,000,000.0		2,800,000		2,800,000	2,700,000
Denominator for dilutive net loss per share (shares)	222,200,000		222,400,000		222,000,000.0	222,300,000
Basic net income (loss) per share (in usd per share)	$ 0.45		$ 0.18		$ 0.75	$ 0.07
Diluted net income (loss) per share (in usd per share)	$ 0.44		$ 0.18		$ 0.74	$ 0.07
Potentially dilutive shares excluded from the computation of diluted net income per share (shares)	100,000		0		0	0