Accumulated Other Comprehensive Income (Loss) - Reclassification out of AOCI (Details) - USD ($) $ in Millions	3 Months Ended
	Aug. 31, 2015	Aug. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Less: Tax effect	$ 30.7	$ 33.8
Net income (loss)	(49.4)	(34.1)
Amount reclassified from Accumulated other comprehensive income (loss) | Retirement benefit plans
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of prior service cost (credit)	0.0	0.0
Amortization of unrecognized gain (loss) included in net periodic cost (credit)	1.0	0.7
Less: Tax effect	(0.3)	(0.2)
Net income (loss)	$ 0.7	$ 0.5