Net Income (Loss) Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended						6 Months Ended
	Jul. 31, 2019	Apr. 30, 2019	Jan. 31, 2019	Oct. 31, 2018	Jul. 31, 2018	Apr. 30, 2018	Jul. 31, 2019	Jul. 31, 2018
Numerator:
Net income (loss)	$ 40.2	$ (24.2)	$ 64.7	$ (23.7)	$ (39.4)	$ (82.4)	$ 16.0	$ (121.8)
Denominator:
Denominator for basic net loss per share—weighted average shares (shares)	219.6				219.0		219.6	218.8
Effect of dilutive securities (shares)	2.8				0.0		2.7	0.0
Denominator for dilutive net loss per share (shares)	222.4				219.0		222.3	218.8
Basic net income (loss) per share (in usd per share)	$ 0.18				$ (0.18)		$ 0.07	$ (0.56)
Diluted net income (loss) per share (in usd per share)	$ 0.18				$ (0.18)		$ 0.07	$ (0.56)
Potentially dilutive shares excluded from the computation of diluted net income per share (shares)					3.2			3.3