Employee Benefit Plans (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	May 31, 2014	May 31, 2013	May 31, 2012
Employee Benefit Plans (Details) [Line Items]
Unrecognized prior service credit	$ 0.3
Percentage of federal subsidy to sponsors of retiree health care benefit plans	28.00%
Cumulative reduction of its accumulated post-retirement benefit obligation	3.1	3.1	2.9
Expected long-term return on plan assets	7.50%
Settlement	1.7
Defined benefit plans tax expense recognized in AOCI	5.0	(8.4)
Estimated employer contributions in 2015	1.4
Employer contributions to defined contribution plan	7.5	8.0	7.4
Annuities [Member] | Level 3 [Member]
Employee Benefit Plans (Details) [Line Items]
Fair value of annuities	6.2	6.1
Pension Plans [Member]
Employee Benefit Plans (Details) [Line Items]
Expected long-term return on plan assets	7.50%	7.30%	7.70%
Settlement	6.4	0
Pension Plans [Member] | Estimated Net Loss [Member]
Employee Benefit Plans (Details) [Line Items]
Net loss amortized from AOCI	1.4
Other Postretirement Benefits [Member]
Employee Benefit Plans (Details) [Line Items]
Expected long-term return on plan assets	0.00%	0.00%	0.00%
Settlement	0	0
Other Postretirement Benefits [Member] | Estimated Net Loss [Member]
Employee Benefit Plans (Details) [Line Items]
Net loss amortized from AOCI	1.6
Other Postretirement Benefits [Member] | Prior Service Credit [Member]
Employee Benefit Plans (Details) [Line Items]
Net loss amortized from AOCI	$ 0.2