Long-Term Debt and Derivatives (Details) (USD $)	3 Months Ended		9 Months Ended			0 Months Ended
	Feb. 28, 2013	Feb. 29, 2012	Feb. 28, 2013	Feb. 29, 2012	May 31, 2012	Jun. 02, 2012 Notes Due 2012 [Member] Senior Notes [Member]	Jun. 05, 2012 Notes Due 2022 [Member] Senior Notes [Member]
Line of Credit Facility [Line Items]
Commercial paper program availability	$ 300,000,000		$ 300,000,000
Revolving credit facility, maximum borrowing capacity with accordion feature	450,000,000		450,000,000
Revolving credit facility amount outstanding	0		0		0
Repayments of Long-term Debt			225,472,000	1,216,000		225,000,000
Debt Instrument, Interest Rate, Stated Percentage						6.00%	3.25%
Proceeds from Issuance of Long-term Debt			250,000,000	0			250,000,000
Other Comprehensive Income Increase from Amortization of Interest Rate Cash Flow Hedge	$ 500,000	$ 400,000	$ 1,500,000	$ 1,100,000