Accumulated Other Comprehensive Income (Loss) - Reclassification out of AOCI (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Feb. 28, 2019	Nov. 30, 2018	Aug. 31, 2018	Feb. 28, 2018	Nov. 30, 2017	Aug. 31, 2017	Feb. 28, 2019	Feb. 28, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of prior service credit	$ 0.0						$ (0.1)
Less: Tax effect	8.2			$ 14.1			3.5	$ 17.4
Net income (loss)	(12.6)	$ 71.6	$ (61.3)	(49.2)	$ 57.1	$ (63.7)	(2.3)	(55.8)
Retirement benefit plans
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of prior service credit	0.0						(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			0.6	1.8
Settlement charge	0.0			39.6			0.0	55.0
Amortization of prior service credit							(0.1)
Less: Tax effect	0.0			(15.9)			0.0	(22.4)
Net income (loss)	$ (0.2)			$ (24.3)			$ (0.5)	$ (34.4)