Long-Term Debt and Derivatives (Details) (USD $)	3 Months Ended		6 Months Ended		0 Months Ended		0 Months Ended
	Nov. 30, 2012	Nov. 30, 2011	Nov. 30, 2012	Nov. 30, 2011	Jun. 02, 2012 Notes Due 2012 [Member] Repayment of Debt [Member] Senior Notes [Member]	Jun. 05, 2012 Notes Due 2012 [Member] Issuance of Debt [Member] Senior Notes [Member]	Jun. 05, 2012 Notes Due 2022 [Member] Issuance of Debt [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
Repayments of Long-term Debt			225,312,000	903,000	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	$ 1,000,000	$ 800,000