Long-Term Debt and Derivatives (Details) (USD $)	12 Months Ended									0 Months Ended		0 Months Ended
	May 31, 2012	May 31, 2011	May 31, 2010	May 31, 2012 Accumulated Other Comprehensive Income (Loss) [Member]	May 31, 2011 Accumulated Other Comprehensive Income (Loss) [Member]	May 31, 2010 Accumulated Other Comprehensive Income (Loss) [Member]	May 31, 2012 Unsecured Term Notes [Member]	May 18, 2011 Notes Due 2016 [Member] Senior Notes [Member]	May 18, 2011 Notes Due 2021 [Member] Senior Notes [Member]	Jun. 02, 2012 Repayment of Debt [Member] Notes Due 2012 [Member] Senior Notes [Member]	Jun. 05, 2012 Issuance of Debt [Member] Notes Due 2012 [Member] Senior Notes [Member]	Jun. 05, 2012 Issuance of Debt [Member] Notes Due 2022 [Member] Senior Notes [Member]
Debt, Derivatives and Hedging Activities
Debt, Weighted Average Interest Rate							5.10%
Letters of credit outstanding, amount	$ 85,700,000	$ 82,700,000
Maturities of long term debt, next twelve months	225,600,000
Maturities of long term debt, year two	8,200,000
Maturities of long term debt, year three	500,000
Maturities of long term debt, year four	200,000
Maturities of long term debt, year five	250,200,000
Interest paid	62,300,000	49,200,000	48,600,000
Commercial paper program availability	300,000,000
Revolving credit facility, maximum borrowing capacity with accordion feature	450,000,000
Debt to EBITDA Ratio Covenant Requirement	3.5
Debt Instrument, Face Amount								250,000,000	250,000,000
Stated interest rate percentage on long term debt								2.85%	4.30%		6.00%	3.25%
Proceeds from issuance of debt	0	1,002,281,000	0									250,000,000
Amortization of interest rate lock agreements	1,508,000	767,000	767,000	1,508,000	767,000	767,000
Payments on long term debt	1,323,000	502,208,000	603,000							225,000,000
Interest rate lock agreement charges	5,800,000
Current accrued liabilities		900,000
Gain (Loss) on Foreign Currency Fair Value Hedge Derivatives	$ (100,000)	$ (300,000)