Loans and Commitments (Details 9) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Feb. 29, 2016	Feb. 28, 2015	Feb. 29, 2016	Feb. 28, 2015	Feb. 29, 2016		May. 31, 2015		Feb. 28, 2015
Activity in the loan loss allowance
Balance at the beginning of the period		$ 39,600	$ 50,757	$ 33,690	$ 56,429
Provision for loan losses		(1,735)	2,304	4,067	(3,475)
Recoveries of loans previously charged-off		53	53	161	160
Balance at the end of the period		37,918	53,114	37,918	53,114
Ending balance of the allowance:
Collectively evaluated						$ 33,364		$ 33,295
Individually evaluated						4,554		395
Total ending balance of the allowance		39,600	50,757	33,690	56,429	37,918		33,690		$ 53,114
Recorded investment in loans:
Collectively evaluated						23,114,418		21,447,484
Individually evaluated						19,797		11,736
Total financing receivables						23,134,215	[1],[2]	21,459,220	[1],[2]	21,202,399
Loans to members, net	[3]					23,096,297		21,425,530
Deferred origination costs						9,884	[1]	9,797	[1]	9,693
CFC
Activity in the loan loss allowance
Balance at the beginning of the period		27,700	41,185	23,716	45,600
Provision for loan losses		(2,136)	2,366	1,740	(2,156)
Recoveries of loans previously charged-off		53	53	161	160
Balance at the end of the period		25,617	43,604	25,617	43,604
Ending balance of the allowance:
Collectively evaluated						25,617		23,716
Individually evaluated								0
Total ending balance of the allowance		27,700	41,185	23,716	45,600	25,617		23,716		43,604
Recorded investment in loans:
Collectively evaluated						22,072,296		20,334,769
Individually evaluated						6,716		7,221
Total financing receivables	[1]					22,079,012		20,341,990
Loans to members, net	[3]					22,053,395		20,318,274
RTFC
Activity in the loan loss allowance
Balance at the beginning of the period		5,918	5,027	4,533	4,282
Provision for loan losses		798	(193)	2,183	552
Balance at the end of the period		6,716	4,834	6,716	4,834
Ending balance of the allowance:
Collectively evaluated						2,162		4,138
Individually evaluated						4,554		395
Total ending balance of the allowance		5,918	5,027	4,533	4,282	6,716		4,533		4,834
Recorded investment in loans:
Collectively evaluated						344,886		381,488
Individually evaluated						13,081		4,221
Total financing receivables	[1]					357,967		385,709
Loans to members, net	[3]					351,251		381,176
NCSC
Activity in the loan loss allowance
Balance at the beginning of the period		5,982	4,545	5,441	6,547
Provision for loan losses		(397)	131	144	(1,871)
Balance at the end of the period		5,585	4,676	5,585	4,676
Ending balance of the allowance:
Collectively evaluated						5,585		5,441
Individually evaluated						0		0
Total ending balance of the allowance		$ 5,982	$ 4,545	$ 5,441	$ 6,547	5,585		5,441		$ 4,676
Recorded investment in loans:
Collectively evaluated						697,236		731,227
Individually evaluated						0		294
Total financing receivables	[1]					697,236		731,521
Loans to members, net	[3]					$ 691,651		$ 726,080

[1] Includes nonperforming and restructured loans.

[2] Represents the unpaid principal balance excluding deferred loan origination costs.

[3] Excludes deferred origination costs of $10 million as of February 29, 2016 and May 31, 2015