Quarterly report pursuant to Section 13 or 15(d)

Short-term Financing (Narrative) (Details)

v3.7.0.1
Short-term Financing (Narrative) (Details)
$ in Millions
 3 Months Ended 9 Months Ended
Feb. 28, 2017
USD ($)
loan
Feb. 28, 2017
USD ($)
loan
Feb. 29, 2016
USD ($)
May 31, 2016
USD ($)
PNC Bank, National Association [Member]        
Line of Credit Facility [Line Items]        
Expiration date   Feb. 28, 2018    
Line of Credit [Member]        
Line of Credit Facility [Line Items]        
Number of lines of credit | loan 4 4    
Amounts outstanding $ 0.0 $ 0.0    
Standby Letters of Credit [Member]        
Line of Credit Facility [Line Items]        
Maximum borrowing capacity 46.8 46.8   $ 43.0
Amounts outstanding 0.0 0.0    
Revolving Credit Facility [Member] | PNC Bank, National Association [Member]        
Line of Credit Facility [Line Items]        
Amounts outstanding 55.4 $ 55.4   $ 0.0
Expiration term   3 years    
Maximum borrowing capacity 150.0 $ 150.0    
Expiration date   Mar. 17, 2019    
Maximum amount borrowed $ 55.6 $ 55.6    
Number of days borrowed 90 days 267 days    
Weighted-average interest rate 1.27% 1.13%    
Revolving Credit Facility [Member] | JP Morgan Chase Bank, NA [Member]        
Line of Credit Facility [Line Items]        
Expiration term   5 years    
Maximum borrowing capacity $ 1,000.0 $ 1,000.0    
Expiration date   Aug. 05, 2020    
Maximum amount borrowed $ 250.0 $ 350.0 $ 350.0  
Number of days borrowed 8 days 27 days 3 days  
Weighted-average interest rate 2.24% 2.68% 3.25%  
Amounts outstanding $ 0.0 $ 0.0    
Revolving Credit Facility [Member] | JP Morgan Chase Bank, NA [Member] | Eight Days [Member]        
Line of Credit Facility [Line Items]        
Maximum amount borrowed $ 50.0      
Number of days borrowed 8 days      
Revolving Credit Facility [Member] | JP Morgan Chase Bank, NA [Member] | Eight Days [Member] | LIBOR [Member]        
Line of Credit Facility [Line Items]        
Weighted-average interest rate 1.44%      
Revolving Credit Facility [Member] | JP Morgan Chase Bank, NA [Member] | Seven Days [Member]        
Line of Credit Facility [Line Items]        
Maximum amount borrowed   $ 150.0    
Number of days borrowed   7 days    
Revolving Credit Facility [Member] | JP Morgan Chase Bank, NA [Member] | Eighteen Days [Member]        
Line of Credit Facility [Line Items]        
Maximum amount borrowed   $ 50.0    
Number of days borrowed   18 days    
Revolving Credit Facility [Member] | JP Morgan Chase Bank, NA [Member] | Eight And Eighteen Days [Member] | LIBOR [Member]        
Line of Credit Facility [Line Items]        
Weighted-average interest rate   1.40%    
Minimum [Member] | Standby Letters of Credit [Member]        
Line of Credit Facility [Line Items]        
Expiration date   Apr. 01, 2017    
Maximum [Member] | Standby Letters of Credit [Member]        
Line of Credit Facility [Line Items]        
Expiration date   Dec. 01, 2017