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Debt (Details) - USD ($)
9 Months Ended
Feb. 29, 2016
May. 31, 2015
Feb. 28, 2015
Debt (Details) [Line Items]      
Debt $ 8,200,000 $ 6,000,000 $ 84,100,000
Loan Agreement      
Debt (Details) [Line Items]      
Borrowing capacity $ 425,000,000    
Expiration date Dec. 05, 2017    
Facility fee (percentage) 0.20%    
Standby letters of credit $ 400,000    
Minimum | Loan Agreement      
Debt (Details) [Line Items]      
Facility fee (percentage) 0.20%    
Maximum | Loan Agreement      
Debt (Details) [Line Items]      
Facility fee (percentage) 0.40%    
Loan Agreement | Federal Funds Rate      
Debt (Details) [Line Items]      
Variable rate (percentage) 0.50%    
Loan Agreement | Eurodollar      
Debt (Details) [Line Items]      
Variable rate (percentage) 1.00%    
Loan Agreement | Eurodollar | Minimum      
Debt (Details) [Line Items]      
Variable rate (percentage) 0.18%    
Loan Agreement | Base Rate | Minimum      
Debt (Details) [Line Items]      
Variable rate (percentage) 0.18%    
Loan Agreement | Base Rate | Maximum      
Debt (Details) [Line Items]      
Variable rate (percentage) 0.60%    
Loan Agreement | London Interbank Offered Rate (LIBOR) | Minimum      
Debt (Details) [Line Items]      
Variable rate (percentage) 1.18%    
Loan Agreement | London Interbank Offered Rate (LIBOR) | Maximum      
Debt (Details) [Line Items]      
Variable rate (percentage) 1.60%    
Unsecured Debt | Domestic Line of Credit      
Debt (Details) [Line Items]      
Borrowing capacity $ 25,000,000    
Outstanding debt $ 0 0 10,400,000
Expiration period (in days) 365 days    
Standby letters of credit $ 4,900,000    
Remaining borrowing capacity 20,100,000    
Unsecured Debt | Foreign Line of Credit      
Debt (Details) [Line Items]      
Borrowing capacity 22,700,000    
Available credit $ 14,500,000    
Expiration period (in days) 364 days    
Unsecured Debt | Foreign Line of Credit | Line of Credit      
Debt (Details) [Line Items]      
Debt $ 8,200,000 $ 6,000,000 $ 8,700,000