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, 2019	Oct. 31, 2018	Jul. 31, 2018	Apr. 30, 2018	Jan. 31, 2018	Oct. 31, 2017	Jul. 31, 2017	Apr. 30, 2017	Jan. 31, 2019	Jan. 31, 2018	Jan. 31, 2017
Numerator:
Net loss	$ 64.7	$ (23.7)	$ (39.4)	$ (82.4)	$ (173.5)	$ (119.8)	$ (144.0)	$ (129.6)	$ (80.8)	$ (566.9)	$ (582.1)
Denominator:
Denominator for basic net loss per share—weighted average shares									218.9	219.5	222.7
Effect of dilutive securities									0.0	0.0	0.0
Denominator for dilutive net loss per share									218.9	219.5	222.7
Basic net loss per share (in dollars per share)	$ 0.30	$ (0.11)	$ (0.18)	$ (0.38)	$ (0.79)	$ (0.55)	$ (0.66)	$ (0.59)	$ (0.37)	$ (2.58)	$ (2.61)
Diluted net loss per share (in dollars per share)	$ 0.29	$ (0.11)	$ (0.18)	$ (0.38)	$ (0.79)	$ (0.55)	$ (0.66)	$ (0.59)	$ (0.37)	$ (2.58)	$ (2.61)
Antidilutive securities (in shares)									3.1	4.5	4.6