- Schedule of activity in goodwill (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Aug. 31, 2018	Aug. 31, 2017	May 31, 2018	May 31, 2017	May 31, 2016
Goodwill [Roll Forward]
Gross goodwill			$ 158.8	$ 158.5	$ 158.5
Accumulated impairment	$ (39.6)	$ (39.6)	(39.6)	$ (39.6)	$ (39.6)
Beginning balance	119.2	118.9	118.9
Foreign currency translation	(0.1)	0.3	0.2
Other	0.0	0.0	0.1
Ending balance	$ 119.1	$ 119.2	$ 119.2