Short-term Financing (Lines of Credit Facility Amount Borrowed) (Details) - Revolving Credit Facility [Member] - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Feb. 28, 2017	Feb. 28, 2017	Feb. 29, 2016
JP Morgan Chase Bank, NA [Member]
Line of Credit Facility [Line Items]
Number of days borrowed	8 days	27 days	3 days
Maximum amount borrowed	$ 250.0	$ 350.0	$ 350.0
Weighted-average amount borrowed	$ 81.3	$ 183.3	$ 217.0
Weighted-average interest rate	2.24%	2.68%	3.25%
PNC Bank, National Association [Member]
Line of Credit Facility [Line Items]
Number of days borrowed	90 days	267 days
Maximum amount borrowed	$ 55.6	$ 55.6
Weighted-average amount borrowed	$ 55.3	$ 52.6
Weighted-average interest rate	1.27%	1.13%