Employee Benefit Plans - Schedule of net periodic costs (Details) - USD ($) $ in Millions	3 Months Ended
	Aug. 31, 2018	Aug. 31, 2017
Components of net periodic benefit (credit) cost:
Amortization of (gain) loss	$ 0.2	$ 0.6
Post-Retirement Benefits
Components of net periodic benefit (credit) cost:
Service cost	0.0	0.0
Interest cost	(0.2)	(0.3)
Expected return on assets	0.0	0.0
Amortization of (gain) loss	0.0	0.0
Net periodic benefit (credit) cost	(0.2)	(0.3)
United States | Pension Plans
Components of net periodic benefit (credit) cost:
Service cost	0.0	0.0
Interest cost	0.0	(0.7)
Expected return on assets	0.0	(1.5)
Amortization of (gain) loss	0.0	0.3
Net periodic benefit (credit) cost	0.0	0.5
United Kingdom | Pension Plans
Components of net periodic benefit (credit) cost:
Service cost	0.0	0.0
Interest cost	(0.2)	(0.3)
Expected return on assets	(0.2)	(0.3)
Amortization of (gain) loss	0.2	0.3
Net periodic benefit (credit) cost	$ (0.2)	$ (0.3)