Net Income (Loss) Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended							9 Months Ended
	Oct. 31, 2019	Jul. 31, 2019	Apr. 30, 2019	Jan. 31, 2019	Oct. 31, 2018	Jul. 31, 2018	Apr. 30, 2018	Oct. 31, 2019	Oct. 31, 2018
Numerator:
Net income (loss)	$ 66.7	$ 40.2	$ (24.2)	$ 64.7	$ (23.7)	$ (39.4)	$ (82.4)	$ 82.7	$ (145.5)
Denominator:
Denominator for basic net loss per share—weighted average shares (shares)	219,700,000				218,900,000			219,600,000	218,700,000
Effect of dilutive securities (shares)	2,200,000				0			2,500,000	0
Denominator for dilutive net loss per share (shares)	221,900,000				218,900,000			222,100,000	218,700,000
Basic net income (loss) per share (in usd per share)	$ 0.30				$ (0.11)			$ 0.38	$ (0.67)
Diluted net income (loss) per share (in usd per share)	$ 0.30				$ (0.11)			$ 0.37	$ (0.67)
Dilutive securities excluded from earnings per share sue to net loss scenario					2,700,000				3,000,000.0
Potentially dilutive shares excluded from the computation of diluted net income per share (shares)	0				700,000			0	300,000