Derivative Financial Instruments (Details 3) (USD $) In Thousands, unless otherwise specified	Feb. 28, 2014	May 31, 2013
Assets:
Net Amount	$ 261,598	$ 257,878
Liabilities:
Net Amount	380,518	475,278
Interest rate swaps
Assets:
Gross amounts of recognized assets	261,598	257,878
Gross amounts offset in the balance sheets	0	0
Net amounts of assets presented in the balance sheets	261,598	257,878
Gross amounts not offset in the balance sheets - Financial instruments	193,733	203,161
Gross amounts not offset in the balance sheets - Cash collateral pledged	0	0
Net Amount	67,865	54,717
Liabilities:
Gross amounts of recognized liabilities	380,518	475,278
Gross amounts offset in the balance sheets	0	0
Net amounts of liabilities presented in the balance sheets	380,518	475,278
Gross amounts not offset in the balance sheets - Financial instruments	193,733	203,161
Gross amounts not offset in the balance sheets - Cash collateral pledged	0	0
Net Amount	$ 186,785	$ 272,117