Accumulated Other Comprehensive Income (Loss) - Reclassification out of AOCI (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Nov. 30, 2018	Nov. 30, 2017	Nov. 30, 2018	Nov. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of prior service credit	$ (0.1)		$ (0.1)
Less: Tax effect	(26.8)	$ (34.6)	(4.7)	$ 3.3
Net income (loss)	71.6	57.1	10.3	(6.6)
Retirement benefit plans
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of prior service credit	(0.1)		(0.1)
Amount reclassified from Accumulated other comprehensive income (loss) | Retirement benefit plans
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of unrecognized gain (loss)	0.2	0.6	0.4	1.2
Settlement charge	0.0	15.4	0.0	15.4
Amortization of prior service credit	(0.1)	0.0	(0.1)	0.0
Less: Tax effect	0.0	(6.3)	0.0	(6.4)
Net income (loss)	$ (0.1)	$ (9.7)	$ (0.3)	$ (10.2)