Accumulated Other Comprehensive Income (Loss) - Reclassification out of AOCI (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Feb. 28, 2015	Feb. 28, 2014	Feb. 28, 2015	Feb. 28, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Less: Tax effect	$ 13.8	$ 28.8	$ (9.7)	$ 12.2
Net income (loss)	(22.1)	(12.1)	12.3	16.3
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)	(0.1)	(0.1)	(0.2)	(0.2)
Amortization of unrecognized gain (loss) included in net periodic cost (credit)	0.8	1.0	2.1	3.1
Settlement charge	0.6	0	4.3	0
Less: Tax effect	(0.5)	0	(2.3)	0
Net income (loss)	$ 0.8	$ 0.9	$ 3.9	$ 2.9