Long-Term Debt and Derivatives (Schedule of Debt) (Details) (USD $) In Thousands, unless otherwise specified	May 31, 2014	May 31, 2013
Debt Instrument [Line Items]
Less: amounts due within one year	$ 503	$ 8,187
Long-term debt due after one year	1,300,477	1,300,979
Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt, weighted average interest rate (percent)	4.60%
Notes Due Through 2036 [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Unsecured term notes due through 2036 at an average rate of 4.6%	1,300,980	1,309,166
Less: amounts due within one year	503	8,187
Long-term debt due after one year	$ 1,300,477	$ 1,300,979