Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended							9 Months Ended
	Oct. 31, 2018	Jul. 31, 2018	Apr. 30, 2018	Jan. 31, 2018	Oct. 31, 2017	Jul. 31, 2017	Apr. 30, 2017	Oct. 31, 2018	Oct. 31, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net loss	$ (23.7)	$ (39.4)	$ (82.4)	$ (173.5)	$ (119.8)	$ (144.0)	$ (129.6)	$ (145.5)	$ (393.4)
Denominator:
Denominator for basic net loss per share—weighted average shares (shares)	218,900,000				219,600,000			218,700,000	219,700,000
Effect of dilutive securities (shares)	0				0			0	0
Denominator for dilutive net loss per share (shares)	218,900,000				219,600,000			218,700,000	219,700,000
Basic net loss per share (in usd per share)	$ (0.11)				$ (0.55)			$ (0.67)	$ (1.79)
Diluted net loss per share (in usd per share)	$ (0.11)				$ (0.55)			$ (0.67)	$ (1.79)
Potentially dilutive shares excluded from the computation of diluted net income per share (shares)	2,700,000				4,100,000			3,000,000.0	4,400,000
Treasury Stock
Denominator:
Potentially dilutive shares excluded from the computation of diluted net income per share (shares)	700,000				0			300,000	300,000