Employee Benefit Plans - Schedule of net periodic costs (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Feb. 28, 2017	Feb. 29, 2016	Feb. 28, 2017	Feb. 29, 2016
Components of net periodic benefit (credit) cost:
Amortization of (gain) loss			$ 3.1	$ 3.3
Pension Plans
Components of net periodic benefit (credit) cost:
Service cost	$ 0.0	$ 0.0	0.0	0.0
Interest cost	1.1	1.5	3.3	4.6
Expected return on assets	(1.8)	(1.9)	(5.4)	(5.8)
Net amortization of prior service credit	0.0	0.0	0.0	0.0
Amortization of (gain) loss	0.4	0.4	1.3	1.2
Net periodic benefit (credit) cost	(0.3)	0.0	(0.8)	0.0
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	0.8	1.0
Expected return on assets	0.0	0.0	0.0	0.0
Net amortization of prior service credit	0.0	0.0	0.0	0.0
Amortization of (gain) loss	(1.0)	0.7	0.2	2.1
Net periodic benefit (credit) cost	$ (0.8)	$ 1.0	$ 1.0	$ 3.1