Quarterly report pursuant to Section 13 or 15(d)

Short-term Financing (Narrative) (Details)

v3.8.0.1
Short-term Financing (Narrative) (Details)
$ in Millions
 3 Months Ended 9 Months Ended
Feb. 28, 2018
USD ($)
Feb. 28, 2017
USD ($)
Feb. 28, 2018
USD ($)
Feb. 28, 2017
USD ($)
Aug. 17, 2017
USD ($)
loan
May 31, 2017
USD ($)
Revolving Credit Facility [Member] | JPM $1 Billion Credit Facility [Member] | JP Morgan Chase Bank, N.A. [Member]            
Line of Credit Facility [Line Items]            
Amounts outstanding $ 0.0   $ 0.0     $ 0.0
Maximum amount borrowed $ 200.0 $ 250.0 $ 700.0 $ 350.0    
Number of days borrowed 2 days 8 days 21 days 27 days    
Weighted-average interest rate 4.25% 2.24% 4.24% 2.68%    
Weighted-average amount borrowed $ 125.0 $ 81.3 $ 311.9 $ 183.3    
Revolving Credit Facility [Member] | JPM $1 Billion Credit Facility [Member] | JP Morgan Chase Bank, N.A. [Member] | Three Days [Member]            
Line of Credit Facility [Line Items]            
Maximum amount borrowed     $ 100.0      
Number of days borrowed     3 days      
Weighted-average interest rate     4.25%      
Revolving Credit Facility [Member] | JPM $1 Billion Credit Facility [Member] | JP Morgan Chase Bank, N.A. [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] | JPM $1 Billion Credit Facility [Member] | JP Morgan Chase Bank, N.A. [Member] | Eight Days [Member] | LIBOR [Member]            
Line of Credit Facility [Line Items]            
Weighted-average interest rate   1.44%        
Revolving Credit Facility [Member] | JPM $1 Billion Credit Facility [Member] | JP Morgan Chase Bank, N.A. [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] | JPM $1 Billion Credit Facility [Member] | JP Morgan Chase Bank, N.A. [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] | JPM $1 Billion Credit Facility [Member] | JP Morgan Chase Bank, N.A. [Member] | Seven And Eighteen Days [Member] | LIBOR [Member]            
Line of Credit Facility [Line Items]            
Weighted-average interest rate       1.40%    
Revolving Credit Facility [Member] | JPM $500 Million Credit Facility [Member] | JP Morgan Chase Bank, N.A. [Member]            
Line of Credit Facility [Line Items]            
Amounts outstanding 0.0   $ 0.0      
Maximum amount borrowed $ 75.0   $ 400.0      
Number of days borrowed 19 days   39 days      
Weighted-average interest rate 2.19%   2.69%      
Weighted-average amount borrowed $ 75.0   $ 147.2      
Revolving Credit Facility [Member] | JPM $500 Million Credit Facility [Member] | JP Morgan Chase Bank, N.A. [Member] | Seven Days [Member]            
Line of Credit Facility [Line Items]            
Maximum amount borrowed     $ 300.0      
Number of days borrowed     7 days      
Revolving Credit Facility [Member] | JPM $500 Million Credit Facility [Member] | JP Morgan Chase Bank, N.A. [Member] | Seven Days [Member] | LIBOR [Member]            
Line of Credit Facility [Line Items]            
Weighted-average interest rate     2.13%      
Revolving Credit Facility [Member] | JPM $500 Million Credit Facility [Member] | JP Morgan Chase Bank, N.A. [Member] | Nineteen Days [Member]            
Line of Credit Facility [Line Items]            
Maximum amount borrowed $ 75.0          
Number of days borrowed 19 days          
Revolving Credit Facility [Member] | JPM $500 Million Credit Facility [Member] | JP Morgan Chase Bank, N.A. [Member] | Nineteen Days [Member] | LIBOR [Member]            
Line of Credit Facility [Line Items]            
Weighted-average interest rate 2.19%          
Revolving Credit Facility [Member] | JPM $500 Million Credit Facility [Member] | JP Morgan Chase Bank, N.A. [Member] | Thirty Days [Member]            
Line of Credit Facility [Line Items]            
Maximum amount borrowed     $ 75.0      
Number of days borrowed     30 days      
Revolving Credit Facility [Member] | JPM $500 Million Credit Facility [Member] | JP Morgan Chase Bank, N.A. [Member] | Thirty Days [Member] | LIBOR [Member]            
Line of Credit Facility [Line Items]            
Weighted-average interest rate     2.19%      
Revolving Credit Facility [Member] | PNC $150 Million Credit Facility [Member] | PNC Bank, National Association [Member]            
Line of Credit Facility [Line Items]            
Amounts outstanding $ 57.7   $ 57.7     0.0
Maximum amount borrowed $ 59.9 $ 55.6 $ 59.9 $ 55.6    
Number of days borrowed 90 days 90 days 269 days 267 days    
Weighted-average interest rate 2.01% 1.27% 1.83% 1.13%    
Weighted-average amount borrowed $ 59.1 $ 55.3 $ 57.6 $ 52.6    
Standby Letters of Credit [Member]            
Line of Credit Facility [Line Items]            
Maximum borrowing capacity 56.8   56.8     47.3
Amounts outstanding 0.0 0.0 0.0 0.0   0.0
Line of Credit [Member]            
Line of Credit Facility [Line Items]            
Number of terminated lines of credit | loan         4  
Amounts outstanding $ 0.0 $ 0.0 $ 0.0 $ 0.0   $ 0.0
Maximum borrowing capacity         $ 900.0  
Minimum [Member] | Standby Letters of Credit [Member]            
Line of Credit Facility [Line Items]            
Expiration Date     Apr. 01, 2018      
Maximum [Member] | Standby Letters of Credit [Member]            
Line of Credit Facility [Line Items]            
Expiration Date     Dec. 01, 2018