Employee Benefit Plans - Schedule of net periodic costs (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Nov. 30, 2019	Nov. 30, 2018	Nov. 30, 2019	Nov. 30, 2018
Components of net periodic benefit (credit) cost:
Amortization of (gain) loss			$ 0.4	$ 0.4
Post-Retirement Benefits
Components of net periodic benefit (credit) cost:
Service cost	$ 0.0	$ 0.0	0.0	0.0
Interest cost	0.1	0.2	0.3	0.4
Expected return on assets	0.0	0.0	0.0	0.0
Net amortization of prior service credit	(0.1)	(0.1)	(0.1)	(0.1)
Amortization of (gain) loss	(0.0)	0.0	0.0	0.0
Net periodic benefit (credit) cost	0.0	0.1	0.2	0.3
United Kingdom | Pension Plans
Components of net periodic benefit (credit) cost:
Service cost	0.0	0.0	0.0	0.0
Interest cost	0.2	0.3	0.4	0.5
Expected return on assets	(0.3)	(0.3)	(0.5)	(0.5)
Net amortization of prior service credit	0.0	0.0	0.0	0.0
Amortization of (gain) loss	0.3	0.2	0.5	0.4
Net periodic benefit (credit) cost	$ 0.2	$ 0.2	$ 0.4	$ 0.4