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
 1 Months Ended 3 Months Ended 6 Months Ended
Dec. 21, 2017
USD ($)
Nov. 30, 2017
USD ($)
Nov. 30, 2016
USD ($)
Nov. 30, 2017
USD ($)
Nov. 30, 2016
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   $ 700.0 $ 350.0 $ 700.0 $ 350.0    
Number of days borrowed   6 days 15 days 19 days 19 days    
Weighted-average interest rate   4.25% 2.53% 4.24% 2.74%    
Weighted-average amount borrowed   $ 445.8 $ 223.3 $ 331.6 $ 226.3    
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] | 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         1.39%    
Revolving Credit Facility [Member] | JPM $1 Billion Credit Facility [Member] | JP Morgan Chase Bank, N.A. [Member] | Two Overnight Basis [Member] | Subsequent Event [Member]              
Line of Credit Facility [Line Items]              
Weighted-average amount borrowed $ 125.0            
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   75.0   $ 75.0      
Expiration Date       Dec. 01, 2017      
Maximum amount borrowed   $ 400.0   $ 400.0      
Number of days borrowed   20 days   20 days      
Weighted-average interest rate   2.84%   2.84%      
Weighted-average amount borrowed   $ 215.8   $ 215.8      
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] | Eleven Days [Member]              
Line of Credit Facility [Line Items]              
Maximum amount borrowed   $ 75.0          
Number of days borrowed   11 days          
Revolving Credit Facility [Member] | JPM $500 Million Credit Facility [Member] | JP Morgan Chase Bank, N.A. [Member] | Eleven 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   $ 58.4   58.4     0.0
Maximum amount borrowed   $ 58.6 $ 53.1 $ 58.6 $ 53.1    
Number of days borrowed   91 days 91 days 179 days 177 days    
Weighted-average interest rate   1.78% 1.08% 1.73% 1.05%    
Weighted-average amount borrowed   $ 57.8 $ 52.2 $ 56.8 $ 51.8    
Standby Letters of Credit [Member]              
Line of Credit Facility [Line Items]              
Maximum borrowing capacity   52.8   52.8     47.3
Amounts outstanding   0.0 0.0 0.0 0.0   $ 0.0
Standby Letters of Credit [Member] | Subsequent Event [Member]              
Line of Credit Facility [Line Items]              
Maximum borrowing capacity $ 4.1            
Expiration Date Nov. 30, 2018            
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    
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