Quarterly report pursuant to Section 13 or 15(d)

Short-term Financing (Narrative) (Details)

v3.6.0.2
Short-term Financing (Narrative) (Details)
$ in Millions
 3 Months Ended 6 Months Ended
Nov. 30, 2016
USD ($)
loan
Nov. 30, 2015
USD ($)
Nov. 30, 2016
USD ($)
loan
Nov. 30, 2015
USD ($)
May 31, 2016
USD ($)
PNC Bank, National Association [Member]          
Line of Credit Facility [Line Items]          
Expiration date     Feb. 27, 2017    
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]          
Line of Credit Facility [Line Items]          
Amounts outstanding 50.0   50.0   0.0
Maximum amount borrowed $ 350.0 $ 350.0 $ 350.0 $ 350.0  
Number of days borrowed 15 days 2 days 19 days 3 days  
Weighted-average interest rate 2.53% 3.25% 2.74% 3.25%  
Revolving Credit Facility [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] | Thirty Days [Member]          
Line of Credit Facility [Line Items]          
Maximum amount borrowed $ 50.0        
Number of days borrowed 30 days        
Revolving Credit Facility [Member] | Seven And Thirty Days [Member] | LIBOR [Member]          
Line of Credit Facility [Line Items]          
Weighted-average interest rate 1.39%        
Revolving Credit Facility [Member] | PNC Bank, National Association [Member]          
Line of Credit Facility [Line Items]          
Amounts outstanding $ 53.1   $ 53.1   $ 0.0
Expiration term     3 years    
Maximum borrowing capacity 150.0   $ 150.0    
Expiration date     Mar. 17, 2019    
Maximum amount borrowed $ 53.1   $ 53.1    
Number of days borrowed 91 days   177 days    
Weighted-average interest rate 1.08%   1.05%    
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    
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