Short-term Financing (Credit Facility Amount Borrowed) (Details) - Revolving Credit Facility [Member] - USD ($) $ in Millions	3 Months Ended
	Aug. 31, 2018	Aug. 31, 2017
JPM $1 Billion Credit Facility [Member] | JP Morgan Chase Bank, N.A. [Member]
Line of Credit Facility [Line Items]
Number of days borrowed	6 days	13 days
Maximum amount borrowed	$ 394.5	$ 660.0
Weighted-average amount borrowed	$ 216.3	$ 278.8
Weighted-average interest rate	4.99%	4.23%
JPM $500 Million Credit Facility [Member] | JP Morgan Chase Bank, N.A. [Member]
Line of Credit Facility [Line Items]
Number of days borrowed	4 days
Maximum amount borrowed	$ 223.0
Weighted-average amount borrowed	$ 133.4
Weighted-average interest rate	4.99%	0.00%
PNC $150 Million Credit Facility [Member] | PNC Bank, National Association [Member]
Line of Credit Facility [Line Items]
Number of days borrowed	88 days	88 days
Maximum amount borrowed	$ 56.8	$ 57.2
Weighted-average amount borrowed	$ 56.0	$ 55.8
Weighted-average interest rate	2.53%	1.67%