Accumulated Other Comprehensive Income (Loss) - Schedule of AOCI Activity (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Nov. 30, 2018	Nov. 30, 2017	Nov. 30, 2018	Nov. 30, 2017
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	$ (58.6)	$ (90.0)	$ (55.7)	$ (94.2)
Other comprehensive income (loss) before reclassifications	(0.6)	0.1	(3.7)	3.8
Benefit from settlement (net of tax of $6.2)		9.2		9.2
Amortization of gains and losses (net of tax of $0.1)	0.2	0.4	0.4	0.9
Postretirement benefit plan remeasurement (net of tax of $0.7)	2.0		2.0
Amortization of prior service credit	(0.1)		(0.1)
Other reclassifications (net of tax of $0.2)	0.6	(0.6)	0.6	(0.6)
Other comprehensive income (loss)	2.1	9.1	(0.8)	13.3
Ending balance	(56.5)	(80.9)	(56.5)	(80.9)
Settlement, Tax		6.2		6.2
Amortization, Tax	0.0	(0.1)	0.0	(0.2)
Remeasurement, Tax	0.7		0.7
Amortization of prior service credit, Tax	0.0		0.0
Other reclassifications, Tax	0.2	0.5	0.2	0.5
Foreign currency translation adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(45.0)	(41.6)	(41.9)	(45.3)
Other comprehensive income (loss) before reclassifications	(0.6)	0.1	(3.7)	3.8
Other comprehensive income (loss)	(0.6)	0.1	(3.7)	3.8
Ending balance	(45.6)	(41.5)	(45.6)	(41.5)
Retirement benefit plans
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(13.6)	(48.4)	(13.8)	(48.9)
Other comprehensive income (loss) before reclassifications	0.0	0.0	0.0	0.0
Benefit from settlement (net of tax of $6.2)		9.2		9.2
Amortization of gains and losses (net of tax of $0.1)	0.2	0.4	0.4	0.9
Postretirement benefit plan remeasurement (net of tax of $0.7)	2.0		2.0
Amortization of prior service credit	(0.1)		(0.1)
Other reclassifications (net of tax of $0.2)	0.6	(0.6)	0.6	(0.6)
Other comprehensive income (loss)	2.7	9.0	2.9	9.5
Ending balance	$ (10.9)	$ (39.4)	$ (10.9)	$ (39.4)