Accumulated Other Comprehensive Income (Loss) - Reclassification out of AOCI (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Nov. 30, 2014	Nov. 30, 2013	Nov. 30, 2014	Nov. 30, 2013
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Less: Tax effect	$ (43.8)	$ (34.3)	$ (23.5)	$ (16.6)
Net income (loss)	68.5	58.3	34.4	28.4
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	(0.1)	(0.1)
Amortization of unrecognized gain (loss) included in net periodic pension cost	0.7	1.1	1.4	2.1
Settlement charge	3.7	0	3.7	0
Less: Tax effect	(1.8)	0	(1.9)	0
Net income (loss)	$ 2.6	$ 1.1	$ 3.1	$ 2.0