Net Income Per Share (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended			6 Months Ended
		Jul. 31, 2016	Apr. 30, 2016	Jul. 31, 2015	Jul. 31, 2016	Jul. 31, 2015
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net loss		$ (98.2)	$ (167.7)	$ (268.6)	$ (265.9)	$ (249.5)
Denominator:
Denominator for basic net loss per share—weighted average shares (shares)		223,200,000		227,000,000	223,800,000	227,100,000
Effect of dilutive securities (shares)	[1]	0		0	0	0
Denominator for dilutive net loss per share (shares)		223,200,000		227,000,000	223,800,000	227,100,000
Basic net loss per share (in usd per share)		$ (0.44)		$ (1.18)	$ (1.19)	$ (1.10)
Diluted net loss per share (in usd per share)		$ (0.44)		$ (1.18)	$ (1.19)	$ (1.10)
Potentially dilutive shares excluded from the computation of diluted net income per share (shares)		4,200,000		4,100,000	4,000,000	4,500,000
Exercise Price Higher than Average Market Value
Denominator:
Potentially dilutive shares excluded from the computation of diluted net income per share (shares)		0		600,000	100,000	100,000

[1] The effect of dilutive securities of 4.2 million shares and 4.1 million shares for the three months ended July 31, 2016 and 2015, respectively, have been excluded from the calculation of diluted net loss per share as those shares would have been anti-dilutive due to the net loss incurred during those periods. The effect of dilutive securities of 4.0 million and 4.5 million shares for the six months ended July 31, 2016 and 2015, respectively, have been excluded from the calculation of diluted net loss per share as those shares would have been anti-dilutive due to the net loss incurred during those periods.