Long-Term Debt and Derivatives (Schedule of Debt) (Details) (USD $) In Thousands, unless otherwise specified	May 31, 2013	May 31, 2012
Debt Instrument [Line Items]
Less: amounts due within one year	$ 8,187	$ 225,636
Long-term debt due after one year	1,300,979	1,059,166
Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt, weighted average interest rate (percent)	4.59%
Notes Due Through 2036 [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Unsecured term notes due through 2036 at an average rate of 4.59%	1,309,166	1,284,802
Less: amounts due within one year	8,187	225,636
Long-term debt due after one year	$ 1,300,979	$ 1,059,166