Short-term Financing (Narrative) (Details) $ in Millions		12 Months Ended
	Jun. 01, 2018 USD ($)	May 31, 2018 USD ($)	May 31, 2017 USD ($)	Aug. 17, 2017 USD ($) loan
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
Maximum amount borrowed		$ 700.0	$ 450.0
Number of days borrowed		22 days	30 days
Weighted-average interest rate		4.27%	2.89%
Weighted-average amount borrowed		$ 319.1	$ 196.7
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			1.40%
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	$ 171.8
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
Maximum amount borrowed		$ 400.0
Number of days borrowed		42 days
Weighted-average interest rate		2.80%
Weighted-average amount borrowed		$ 144.8
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] | 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] | JPM $500 Million Credit Facility [Member] | JP Morgan Chase Bank, N.A. [Member] | Three Overnight Basis [Member] | Subsequent Event [Member]
Line of Credit Facility [Line Items]
Weighted-average amount borrowed	103.5
Revolving Credit Facility [Member] | PNC $150 Million Credit Facility [Member] | PNC Bank, National Association [Member]
Line of Credit Facility [Line Items]
Amounts outstanding		$ 0.0	$ 0.0
Maximum amount borrowed		$ 59.9	$ 55.6
Number of days borrowed		358 days	353 days
Weighted-average interest rate		1.94%	1.21%
Weighted-average amount borrowed		$ 57.2	$ 52.8
Revolving Credit Facility [Member] | PNC $150 Million Credit Facility [Member] | PNC Bank, National Association [Member] | Subsequent Event [Member]
Line of Credit Facility [Line Items]
Amounts outstanding	56.8
Standby Letters of Credit [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity		56.8	47.3
Amounts outstanding		0.0	0.0
Standby Letters of Credit [Member] | Extended Maturity [Member] | Subsequent Event [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 60.9
Expiration Date	Jul. 01, 2019
Increase amount available	$ 4.1
Line of Credit [Member]
Line of Credit Facility [Line Items]
Number of terminated lines of credit | loan				4
Amounts outstanding		$ 0.0	$ 0.0
Maximum borrowing capacity				$ 900.0
Minimum [Member] | Standby Letters of Credit [Member]
Line of Credit Facility [Line Items]
Expiration Date		Jul. 01, 2018
Maximum [Member] | Standby Letters of Credit [Member]
Line of Credit Facility [Line Items]
Expiration Date		May 01, 2019