Long-Term Debt and Derivatives - Schedule of Debt (Details) - Term Notes Due Through 2036 - Unsecured Debt - USD ($) $ in Thousands	May 31, 2016	May 31, 2015
Debt Instrument [Line Items]
Unsecured term notes due through 2036 at an average rate of 4.6%	$ 1,300,000	$ 1,300,000
Less: amounts due within one year	250,000	0
Long-term debt due after one year	$ 1,050,000	$ 1,300,000
Average interest rate	4.60%	4.60%