Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Apr. 30, 2019	Jan. 31, 2019	Oct. 31, 2018	Jul. 31, 2018	Apr. 30, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net loss	$ (24.2)	$ 64.7	$ (23.7)	$ (39.4)	$ (82.4)
Denominator:
Denominator for basic net loss per share—weighted average shares (shares)	219,600,000				218,600,000
Effect of dilutive securities (shares)	0				0
Denominator for dilutive net loss per share (shares)	219,600,000				218,600,000
Basic net loss per share (in usd per share)	$ (0.11)				$ (0.38)
Diluted net loss per share (in usd per share)	$ (0.11)				$ (0.38)
Potentially dilutive shares excluded from the computation of diluted net income per share (shares)	2,300,000				3,000,000.0
Treasury Stock
Denominator:
Potentially dilutive shares excluded from the computation of diluted net income per share (shares)	0				200,000