Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Feb. 28, 2014	May 31, 2013
Assets:
Restricted cash	$ 7,256	$ 7,696
Time deposits	700,000	700,000
Debt service reserve funds	39,353	39,803
Derivative instruments	261,598	257,878
Liabilities:
Short-term debt	6,541,987	7,719,483
Long-term debt	12,902,920	10,696,433
Derivative instruments	380,518	475,278
Subordinated deferrable debt
Liabilities:
Subordinated debt	400,000	400,000
Members' subordinated certificates
Liabilities:
Subordinated debt	1,486,965	1,729,226
Carrying value
Assets:
Cash and cash equivalents	842,915	177,062
Restricted cash	7,256	7,696
Investments	28,702	31,632
Time deposits	700,000	700,000
Deferred compensation investments	4,035	3,716
Loans to members, net	20,589,024	20,251,549
Debt service reserve funds	39,353	39,803
Derivative instruments	261,598	257,878
Liabilities:
Short-term debt	6,541,987	7,719,483
Long-term debt	12,902,920	10,696,433
Guarantee liability	23,172	24,742
Derivative instruments	380,518	475,278
Off-balance sheet instruments:
Commitments	0	0
Carrying value | Subordinated deferrable debt
Liabilities:
Subordinated debt	400,000	400,000
Carrying value | Members' subordinated certificates
Liabilities:
Subordinated debt	1,486,965	1,729,226
Fair value
Assets:
Cash and cash equivalents	842,915	177,062
Restricted cash	7,256	7,696
Investments	28,702	31,632
Time deposits	700,000	700,000
Deferred compensation investments	4,035	3,716
Loans to members, net	21,015,784	21,318,406
Debt service reserve funds	39,353	39,803
Derivative instruments	261,598	257,878
Liabilities:
Short-term debt	6,547,673	7,751,021
Long-term debt	14,079,914	12,156,097
Guarantee liability	26,061	27,730
Derivative instruments	380,518	475,278
Off-balance sheet instruments:
Commitments	0	0
Fair value | Subordinated deferrable debt
Liabilities:
Subordinated debt	372,200	404,300
Fair value | Members' subordinated certificates
Liabilities:
Subordinated debt	$ 1,486,965	$ 1,880,672