Long-Term Debt and Derivatives - Narrative (Details) - USD ($)		1 Months Ended	12 Months Ended
	Jun. 01, 2016	Jun. 30, 2016	May 31, 2016	May 31, 2015	May 31, 2014	Feb. 29, 2016
Debt Instrument [Line Items]
Letters of credit outstanding, amount			$ 83,400,000	$ 82,700,000
Interest paid			64,500,000	65,300,000	$ 65,900,000
Commercial paper program availability			300,000,000
Revolving credit facility, maximum borrowing capacity with accordion feature			600,000,000
Proceeds from issuance of commercial paper	$ 218,500,000
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			250,000,000
Maturities of long term debt, year two			300,000,000
Maturities of long term debt, year three			0
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
Treasury Lock
Debt Instrument [Line Items]
Interest rate lock agreement, notional value						$ 550,000,000
Treasury Lock | Other Noncurrent Liabilities
Debt Instrument [Line Items]
Interest rate lock agreement, fair value			19,600,000
Term Notes Due Through 2036 | Unsecured Debt
Debt Instrument [Line Items]
Unsecured term notes due through 2036 at an average rate of 4.6%			1,300,000,000	1,300,000,000
Long-term debt, fair value			1,416,600,000	$ 1,418,600,000
Debt Amendment June 2016 | Revolving Credit Facility
Debt Instrument [Line Items]
Revolving credit facility, maximum borrowing capacity with accordion feature			$ 450,000,000.0
Subsequent Event | Term Notes Due Through 2036 | Unsecured Debt
Debt Instrument [Line Items]
Refinanced debt	$ 250,000,000
Term of debt		5 years