Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Oct. 31, 2017	Oct. 31, 2016	Oct. 31, 2017	Oct. 31, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net loss	$ (119.8)	$ (142.8)	$ (393.4)	$ (408.7)
Denominator:
Denominator for basic net loss per share—weighted average shares (shares)	219,600,000	222,300,000	219,700,000	223,300,000
Effect of dilutive securities (shares)	0	0	0	0
Denominator for dilutive net loss per share (shares)	219,600,000	222,300,000	219,700,000	223,300,000
Basic net loss per share (in usd per share)	$ (0.55)	$ (0.64)	$ (1.79)	$ (1.83)
Diluted net loss per share (in usd per share)	$ (0.55)	$ (0.64)	$ (1.79)	$ (1.83)
Potentially dilutive shares excluded from the computation of diluted net income per share (shares)	4,100,000	4,400,000	4,400,000	4,000,000.0
Exercise Price Higher than Average Market Value
Denominator:
Potentially dilutive shares excluded from the computation of diluted net income per share (shares)	0	0	300,000	400,000