Net Income (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, 2020	Oct. 31, 2019	Jul. 31, 2019	Apr. 30, 2019	Jan. 31, 2019	Oct. 31, 2018	Jul. 31, 2018	Apr. 30, 2018	Jan. 31, 2020	Jan. 31, 2019	Jan. 31, 2018
Numerator:
Net income (loss)	$ 131.8	$ 66.7	$ 40.2	$ (24.2)	$ 64.7	$ (23.7)	$ (39.4)	$ (82.4)	$ 214.5	$ (80.8)	$ (566.9)
Denominator:
Denominator for basic net income (loss) per share—weighted average shares									219.7	218.9	219.5
Effect of dilutive securities									2.8	0.0	0.0
Denominator for dilutive net income (loss) per share									222.5	218.9	219.5
Basic net income (loss) per share (in dollars per share)	$ 0.60	$ 0.30	$ 0.18	$ (0.11)	$ 0.30	$ (0.11)	$ (0.18)	$ (0.38)	$ 0.98	$ (0.37)	$ (2.58)
Diluted net income (loss) per share (in dollars per share)	$ 0.59	$ 0.30	$ 0.18	$ (0.11)	$ 0.29	$ (0.11)	$ (0.18)	$ (0.38)	$ 0.96	$ (0.37)	$ (2.58)
Antidilutive securities (in shares)										3.1	4.5