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Employee Benefit Plans (Details) - USD ($)
 12 Months Ended
May 31, 2016
May 31, 2015
May 31, 2014
Employee Benefit Plans (Details) [Line Items]      
Unrecognized prior service credit $ 0    
Percentage of federal subsidy to sponsors of retiree health care benefit plans 28.00%    
Cumulative reduction of its accumulated post-retirement benefit obligation $ 3,100,000 $ 3,000,000 $ 3,100,000
Expected long-term return on plan assets 4.70%    
Settlement   4,300,000  
Defined benefit plans tax expense recognized in AOCI $ 1,800,000 (2,500,000) (5,000,000)
Estimated employer contributions in 2015 1,200,000    
Employer contributions to defined contribution plan 6,800,000 7,900,000 $ 7,500,000
Annuities [Member] | Level 3 [Member]      
Employee Benefit Plans (Details) [Line Items]      
Fair value of annuities $ 5,500,000 $ 6,100,000  
Pension Plans [Member]      
Employee Benefit Plans (Details) [Line Items]      
Expected long-term return on plan assets 4.70% 5.40% 7.50%
Settlement $ 0 $ 14,400,000  
Estimated net loss to be recognized over next twelve months $ 1,800,000    
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  
Estimated net loss to be recognized over next twelve months 2,400,000    
Change in Accounting Method Accounted for as Change in Estimate [Member] | Pension Plans [Member]      
Employee Benefit Plans (Details) [Line Items]      
Defined Benefit Plan, Interest and Service Cost 0    
Change in Accounting Method Accounted for as Change in Estimate [Member] | Other Postretirement Benefits [Member]      
Employee Benefit Plans (Details) [Line Items]      
Defined Benefit Plan, Interest and Service Cost $ 0