Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jul. 31, 2017	Jul. 31, 2016	Jul. 31, 2017	Jul. 31, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net loss	$ (144.0)	$ (98.2)	$ (273.6)	$ (265.9)
Denominator:
Denominator for basic net loss per share—weighted average shares (shares)	219,500,000	223,200,000	219,700,000	223,800,000
Effect of dilutive securities (shares)	0	0	0	0
Denominator for dilutive net loss per share (shares)	219,500,000	223,200,000	219,700,000	223,800,000
Basic net loss per share (in usd per share)	$ (0.66)	$ (0.44)	$ (1.25)	$ (1.19)
Diluted net loss per share (in usd per share)	$ (0.66)	$ (0.44)	$ (1.25)	$ (1.19)
Potentially dilutive shares excluded from the computation of diluted net income per share (shares)	4,800,000	4,200,000	4,700,000	4,000,000
Exercise Price Higher than Average Market Value
Denominator:
Potentially dilutive shares excluded from the computation of diluted net income per share (shares)	0	0	100,000	100,000