Net Income Per Share - Computation of Net Income Per Share Amounts (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended				6 Months Ended
	Jul. 31, 2021	Apr. 30, 2021	Jul. 31, 2020	Apr. 30, 2020	Jul. 31, 2021	Jul. 31, 2020
Numerator:
Net income	$ 115.6	$ 155.6	$ 98.2	$ 66.5	$ 271.2	$ 164.7
Denominator:
Denominator for basic net income per share—weighted average shares (in shares)	219.8		219.2		219.7	219.2
Effect of dilutive securities (in shares)	2.7		3.0		2.5	2.8
Denominator for dilutive net income per share (in shares)	222.5		222.2		222.2	222.0
Basic net income per share (in usd per share)	$ 0.53		$ 0.45		$ 1.23	$ 0.75
Diluted net income per share (in usd per share)	$ 0.52		$ 0.44		$ 1.22	$ 0.74