Income Taxes - Provision for income taxes (Details) - USD ($) $ 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
Federal:
Current									$ (0.8)	$ 1.6	$ (4.7)
Deferred									(19.3)	8.4	220.9
State:
Current									(0.3)	(1.9)	0.5
Deferred									2.2	1.3	20.9
Foreign:
Current									50.9	93.9	68.4
Deferred									(23.1)	(45.0)	4.2
Income tax expense	$ (24.8)	$ 8.6	$ 17.6	$ 8.2	$ 5.2	$ 13.5	$ 25.2	$ 14.4	$ 9.6	$ 58.3	$ 310.2