Long-Term Debt and Derivatives - Narrative (Details) - USD ($)	12 Months Ended
	May. 31, 2015	May. 31, 2014	May. 31, 2013
Debt Instrument [Line Items]
Letters of credit outstanding, amount	$ 82,700,000	$ 85,100,000
Interest paid	65,300,000	65,900,000	$ 68,400,000
Commercial paper program availability	300,000,000
Revolving credit facility, maximum borrowing capacity with accordion feature	450,000,000
Commercial paper outstanding	0	0
Borrowings on revolving credit facility	0	0
Long-term Debt, Fiscal Year Maturity [Abstract]
Maturities of long term debt, next twelve months	0
Maturities of long term debt, year two	250,000,000
Maturities of long term debt, year three	300,000,000
Maturities of long term debt, year four	0
Maturities of long term debt, year five	0
Other comprehensive Income
Debt Instrument [Line Items]
Amortization of interest rate lock agreements	2,000,000	2,000,000	$ 2,000,000
Term Notes Due Through 2036 | Unsecured Debt
Debt Instrument [Line Items]
Long-term debt, carrying amount	1,300,000,000	1,300,980,000
Long-term debt, fair value	$ 1,418,600,000	1,421,000,000
Foreign Currency Forward Contracts
Debt Instrument [Line Items]
Current accrued liabilities		$ 300,000