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Debt - Narrative (Details) - USD ($)
3 Months Ended
Aug. 31, 2017
May 31, 2017
Aug. 31, 2016
Debt (Details) [Line Items]      
Standby letters of credit $ 5,300,000    
Debt 12,000,000 $ 6,200,000 $ 12,100,000
Loan Agreement      
Debt (Details) [Line Items]      
Borrowing capacity 375,000,000    
Debt limit before equity transaction restrictions apply $ 275,000,000    
Expiration date Jan. 05, 2022    
Facility fee (percentage) 0.20%    
Increase in borrwoing capacity available under accordion feature $ 150,000,000.0    
Standby letters of credit $ 400,000    
Loan Agreement | Minimum      
Debt (Details) [Line Items]      
Facility fee (percentage) 0.20%    
Loan Agreement | Maximum      
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 | Base Rate      
Debt (Details) [Line Items]      
Variable rate (percentage) 0.175%    
Loan Agreement | Base Rate | Minimum      
Debt (Details) [Line Items]      
Variable rate (percentage) 0.175%    
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) 1.175%    
Loan Agreement | London Interbank Offered Rate (LIBOR) | Maximum      
Debt (Details) [Line Items]      
Variable rate (percentage) 1.60%    
2007 Loan Agreement      
Debt (Details) [Line Items]      
Borrowing capacity $ 425,000,000    
Debt limit before equity transaction restrictions apply 75,000,000    
Unsecured Debt | Domestic Line of Credit      
Debt (Details) [Line Items]      
Borrowing capacity 25,000,000    
Standby letters of credit 4,900,000    
Short-term debt 0 0 0
Remaining borrowing capacity $ 20,100,000    
Expiration period (in days) 365 days    
Secured Debt | Line of Credit      
Debt (Details) [Line Items]      
Borrowing capacity $ 24,200,000    
Expiration period (in days) 364 days    
Debt $ 12,000,000 $ 6,200,000 $ 12,100,000
Weighted average interest rate 3.80% 4.10% 4.10%
Available credit $ 12,200,000    
Revolving Credit Facility | 2007 Loan Agreement | Letter of Credit      
Debt (Details) [Line Items]      
Borrowing capacity 50,000,000    
Revolving Credit Facility | 2007 Loan Agreement | Swingline Facility      
Debt (Details) [Line Items]      
Borrowing capacity $ 15,000,000