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Employee Benefit Plans (Details) - USD ($)
$ in Millions
 12 Months Ended
May. 31, 2015
May. 31, 2014
May. 31, 2013
Employee Benefit Plans (Details) [Line Items]      
Unrecognized prior service credit $ 0.1    
Percentage of federal subsidy to sponsors of retiree health care benefit plans 28.00%    
Cumulative reduction of its accumulated post-retirement benefit obligation $ 3.0 $ 3.1 $ 3.1
Expected long-term return on plan assets 5.40%    
Settlement $ 4.3 1.7  
Defined benefit plans tax expense recognized in AOCI 2.5 (5.0)  
Estimated employer contributions in 2015 1.3    
Employer contributions to defined contribution plan 7.9 7.5 $ 8.0
Annuities [Member] | Level 3 [Member]      
Employee Benefit Plans (Details) [Line Items]      
Fair value of annuities $ 6.1 $ 6.2  
Pension Plans [Member]      
Employee Benefit Plans (Details) [Line Items]      
Expected long-term return on plan assets 5.40% 7.50% 7.30%
Settlement $ 14.4 $ 6.4  
Estimated net loss to be recognized over next twelve months $ 1.7    
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 $ 0.0  
Estimated net loss to be recognized over next twelve months 2.2    
Prior service credit to be recognized over next twelve months $ 0.1