Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jul. 31, 2018	Jul. 31, 2017	Jul. 31, 2018	Jul. 31, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net loss	$ (39.4)	$ (144.0)	$ (121.8)	$ (273.6)
Denominator:
Denominator for basic net loss per share—weighted average shares (shares)	219,000,000.0	219,500,000	218,800,000	219,700,000
Effect of dilutive securities (shares)	0	0	0	0
Denominator for dilutive net loss per share (shares)	219,000,000.0	219,500,000	218,800,000	219,700,000
Basic net loss per share (in usd per share)	$ (0.18)	$ (0.66)	$ (0.56)	$ (1.25)
Diluted net loss per share (in usd per share)	$ (0.18)	$ (0.66)	$ (0.56)	$ (1.25)
Potentially dilutive shares excluded from the computation of diluted net income per share (shares)	3,200,000	4,800,000	3,300,000	4,700,000
Treasury Stock
Denominator:
Potentially dilutive shares excluded from the computation of diluted net income per share (shares)	0	0	0	100,000