Net Loss Per Share - Computation of Net Income Per Share Amounts (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Jan. 31, 2018	Oct. 31, 2017	Jul. 31, 2017	Apr. 30, 2017	Jan. 31, 2017	Oct. 31, 2016	Jul. 31, 2016	Apr. 30, 2016	Jan. 31, 2018	Jan. 31, 2017	Jan. 31, 2016
Numerator:
Net loss	$ (173.5)	$ (119.8)	$ (144.0)	$ (129.6)	$ (173.4)	$ (142.8)	$ (98.2)	$ (167.7)	$ (566.9)	$ (582.1)	$ (330.5)
Denominator:
Denominator for basic net loss per share—weighted average shares									219.5	222.7	226.0
Effect of dilutive securities									0.0	0.0	0.0
Denominator for dilutive net loss per share									219.5	222.7	226.0
Basic net loss per share (usd per share)	$ (0.79)	$ (0.55)	$ (0.66)	$ (0.59)	$ (0.78)	$ (0.64)	$ (0.44)	$ (0.75)	$ (2.58)	$ (2.61)	$ (1.46)
Diluted net loss per share (usd per share)	$ (0.79)	$ (0.55)	$ (0.66)	$ (0.59)	$ (0.78)	$ (0.64)	$ (0.44)	$ (0.75)	$ (2.58)	$ (2.61)	$ (1.46)
Antidilutive securities (in shares)									4.5	4.6	4.7