Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Feb. 29, 2016	May. 31, 2015	Feb. 28, 2015	May. 31, 2014
Fair value of Financial Instruments
Cash and cash equivalents	$ 299,024	$ 248,836	$ 298,771	$ 338,715
Assets:
Restricted cash	4,268	485
Investment securities available for sale, at fair value	85,061	84,472
Deferred compensation investments	3,953	4,294
Loans to members, net	23,106,181	21,435,327	$ 21,158,978
Accrued interest and other receivables	179,212	197,828
Debt service reserve funds	17,151	25,602
Derivative instruments	104,535	115,276
Short-term debt	3,309,020	3,127,754
Liabilities:
Long-term debt	17,527,497	16,244,794
Accrued interest payable	185,758	123,697
Guaranty Liabilities Contingent and Noncontingent	18,155	19,917
Derivative instruments	688,998	408,382
Subordinated deferrable debt	395,754	395,699
Members Subordinated Certificates, Total	1,444,515	1,505,420
Level 1
Assets:
Cash and cash equivalents	299,024	248,836
Restricted cash	4,268	485
Investments	85,061	84,472
Deferred compensation investments	3,953	4,294
Accrued interest and other receivables	179,212	197,828
Debt service reserve funds	17,151	25,602
Liabilities:
Short-term debt	1,499,503	1,494,131
Accrued interest payable	185,758	123,697
Level 2
Assets:
Time deposits	340,000	485,000
Derivative instruments	104,535	115,276
Liabilities:
Short-term debt	1,809,014	1,633,410
Long-term debt	11,311,512	10,878,302
Derivative instruments	688,998	408,382
Level 3
Assets:
Loans to members, net	23,424,554	21,961,048
Liabilities:
Short-term debt	0	0
Long-term debt	7,299,684	6,477,921
Guarantee liability	20,144	22,545
Members Subordinated Certificates, At Fair Value	1,444,539	1,505,444
Subordinated deferrable debt | Level 2
Liabilities:
Subordinated deferrable debt	389,220	406,000
Subordinated certificates
Liabilities:
Members Subordinated Certificates, Total	1,444,515	1,505,420
Fair value
Assets:
Cash and cash equivalents	299,024	248,836
Restricted cash	4,268	485
Investments	85,061	84,472
Time deposits	340,000
Loans to members, net	23,424,554	21,961,048
Accrued interest and other receivables	179,212	197,828
Debt service reserve funds	17,151	25,602
Derivative instruments	104,535	115,276
Short-term debt		3,127,541
Liabilities:
Short-term debt	3,308,517
Long-term Debt, Fair Value	18,611,196	17,356,223
Accrued interest payable	185,758	123,697
Guarantee liability	20,144	22,545
Derivative instruments	688,998	408,382
Fair value | Subordinated deferrable debt
Liabilities:
Subordinated deferrable debt	389,220	406,000
Fair value | Subordinated certificates
Liabilities:
Members Subordinated Certificates, At Fair Value	1,444,539	1,505,444
Recurring basis | Level 1
Assets:
Investment securities available for sale, at fair value	85,061	84,472
Deferred compensation investments	3,953	4,294
Interest rate swaps
Assets:
Fair value of derivative assets	104,535	115,276
Derivative instruments	104,535
Liabilities:
Derivative liabilities	688,998	408,382
Derivative instruments	688,998
Interest rate swaps | Level 2
Assets:
Derivative instruments	104,535	115,276
Liabilities:
Derivative instruments	$ 688,998	$ 408,382