Long-Term Debt and Derivatives - Schedule of Debt (Details) - USD ($) $ in Thousands	May. 31, 2015	May. 31, 2014
Debt Instrument [Line Items]
Less: amounts due within one year	$ 0	$ 503
Long-term debt due after one year	1,300,000	1,300,477
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	1,300,980
Less: amounts due within one year	0	503
Long-term debt due after one year	$ 1,300,000	$ 1,300,477
Average interest rate	4.60%	4.60%