Employee Benefit Plans - Schedule of net periodic costs (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Nov. 30, 2017	Nov. 30, 2016	Nov. 30, 2017	Nov. 30, 2016
Components of net periodic benefit (credit) cost:
Amortization of (gain) loss			$ 1.2	$ 2.1
Post-Retirement Benefits
Components of net periodic benefit (credit) cost:
Service cost	$ 0.0	$ 0.0	0.0	0.0
Interest cost	0.2	0.3	0.5	0.6
Expected return on assets	0.0	0.0	0.0	0.0
Benefit cost of settlement event	0.0	0.0	0.0	0.0
Amortization of (gain) loss	0.0	0.6	0.0	1.2
Net periodic benefit (credit) cost	0.2	0.9	0.5	1.8
United States | Pension Plans
Components of net periodic benefit (credit) cost:
Service cost	0.0	0.0	0.0	0.0
Interest cost	0.8	0.8	1.5	1.6
Expected return on assets	(1.5)	(1.6)	(3.0)	(3.1)
Benefit cost of settlement event	15.4	0.0	15.4	0.0
Amortization of (gain) loss	0.3	0.3	0.6	0.5
Net periodic benefit (credit) cost	15.0	(0.5)	14.5	(1.0)
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	0.5	0.6
Expected return on assets	(0.2)	(0.2)	(0.5)	(0.5)
Benefit cost of settlement event	0.0	0.0	0.0	0.0
Amortization of (gain) loss	0.3	0.2	0.6	0.4
Net periodic benefit (credit) cost	$ 0.3	$ 0.3	$ 0.6	$ 0.5