Employee Benefit Plans - Schedule of net periodic costs (Details) - USD ($) $ in Millions	3 Months Ended
	Aug. 31, 2017	Aug. 31, 2016
Components of net periodic benefit (credit) cost:
Amortization of (gain) loss	$ 0.6	$ 1.0
Post-Retirement Benefits
Components of net periodic benefit (credit) cost:
Service cost	0.0	0.0
Interest cost	0.3	0.3
Expected return on assets	0.0	0.0
Amortization of (gain) loss	0.0	0.6
Net periodic benefit (credit) cost	0.3	0.9
United States | Pension Plans
Components of net periodic benefit (credit) cost:
Service cost	0.0	0.0
Interest cost	0.7	0.8
Expected return on assets	(1.5)	(1.5)
Amortization of (gain) loss	0.3	0.2
Net periodic benefit (credit) cost	(0.5)	(0.5)
United Kingdom | Pension Plans
Components of net periodic benefit (credit) cost:
Service cost	0.0	0.0
Interest cost	0.3	0.3
Expected return on assets	(0.3)	(0.3)
Amortization of (gain) loss	0.3	0.2
Net periodic benefit (credit) cost	$ 0.3	$ 0.2