Business Segments - Segment Results and Total Assets (Details) $ in Thousands	3 Months Ended
	Aug. 31, 2022 USD ($) segment	Aug. 31, 2021 USD ($)	May 31, 2022 USD ($)	May 31, 2021 USD ($)
Segment Information
Number of operating segments | segment	3
Statement of operations:
Interest income	$ 306,978	$ 283,268
Interest expense	(209,468)	(174,777)
Derivative cash settlements interest expense	0	0
Interest expense	(209,468)	(174,777)
Net interest income	97,510	108,491
Provision for credit losses	(3,496)	(4,003)
Net interest income after provision for credit losses	94,014	104,488
Non-interest income:
Fee and other income	4,056	3,941
Derivative forward value gains (losses)	104,372	(144,600)
Derivative gains (losses)	93,587	(172,163)
Investment securities losses	(3,679)	(2,225)
Total non-interest income	93,964	(170,447)
Non-interest expense:
General and administrative expenses	(25,519)	(24,210)
Other non-interest expense	(322)	(256)
Total non-interest expense	(25,841)	(24,466)
Income (loss) before income taxes	162,137	(90,425)
Income tax benefit (provision)	(263)	93
Net income (loss)	161,874	(90,332)
Assets:
Total loans outstanding	30,674,996	28,870,758	$ 30,051,354
Deferred loan origination costs	12,335	11,875
Loans to members	30,687,331	28,882,633	30,063,386
Less: Allowance for credit losses	(71,056)	(89,535)	(67,560)	$ (85,532)
Loans to members, net	30,616,275	28,793,098	29,995,826
Other assets	1,466,019	1,298,649
Total assets	32,082,294	30,091,747	$ 31,251,382
Interest rate swaps
Statement of operations:
Interest expense	(10,785)	(27,563)
Derivative cash settlements interest expense	(10,785)	(27,563)
Non-interest income:
Derivative forward value gains (losses)	104,372	(144,600)
Derivative gains (losses)	93,587	(172,163)
Operating segments
Statement of operations:
Interest income	316,920	291,760
Interest expense	(219,410)	(183,269)
Derivative cash settlements interest expense	(10,785)	(27,563)
Interest expense	(230,195)	(210,832)
Net interest income	86,725	80,928
Provision for credit losses	(3,743)	(4,209)
Net interest income after provision for credit losses	82,982	76,719
Non-interest income:
Fee and other income	6,712	6,207
Derivative gains (losses)	0	0
Investment securities losses	(3,679)	(2,225)
Total non-interest income	3,033	3,982
Non-interest expense:
General and administrative expenses	(27,547)	(25,805)
Other non-interest expense	(703)	(721)
Total non-interest expense	(28,250)	(26,526)
Income (loss) before income taxes	57,765	54,175
Income tax benefit (provision)	(263)	93
Net income (loss)	57,502	54,268
Assets:
Total loans outstanding	31,853,805	29,987,128
Deferred loan origination costs	12,335	11,875
Loans to members	31,866,140	29,999,003
Less: Allowance for credit losses	(74,038)	(89,535)
Loans to members, net	31,792,102	29,909,468
Other assets	1,553,989	1,387,636
Total assets	33,346,091	31,297,104
Operating segments | Interest rate swaps
Statement of operations:
Interest expense	0	0
Non-interest income:
Derivative forward value gains (losses)	0	0
Reclasses and adjustments
Statement of operations:
Interest income	0	0
Interest expense	0	0
Derivative cash settlements interest expense	10,785	27,563
Interest expense	10,785	27,563
Net interest income	10,785	27,563
Provision for credit losses	0	0
Net interest income after provision for credit losses	10,785	27,563
Non-interest income:
Fee and other income	0	0
Derivative gains (losses)	93,587	(172,163)
Investment securities losses	0	0
Total non-interest income	93,587	(172,163)
Non-interest expense:
General and administrative expenses	0	0
Other non-interest expense	0	0
Total non-interest expense	0	0
Income (loss) before income taxes	104,372	(144,600)
Income tax benefit (provision)	0	0
Net income (loss)	104,372	(144,600)
Assets:
Total loans outstanding	0	0
Deferred loan origination costs	0	0
Loans to members	0	0
Less: Allowance for credit losses	0	0
Loans to members, net	0	0
Other assets	0	0
Total assets	0	0
Reclasses and adjustments | Interest rate swaps
Statement of operations:
Interest expense	(10,785)	(27,563)
Non-interest income:
Derivative forward value gains (losses)	104,372	(144,600)
Intersegment eliminations
Statement of operations:
Interest income	(9,942)	(8,492)
Interest expense	9,942	8,492
Derivative cash settlements interest expense	0	0
Interest expense	9,942	8,492
Net interest income	0	0
Provision for credit losses	247	206
Net interest income after provision for credit losses	247	206
Non-interest income:
Fee and other income	(2,656)	(2,266)
Derivative gains (losses)	0	0
Investment securities losses	0	0
Total non-interest income	(2,656)	(2,266)
Non-interest expense:
General and administrative expenses	2,028	1,595
Other non-interest expense	381	465
Total non-interest expense	2,409	2,060
Income (loss) before income taxes	0	0
Income tax benefit (provision)	0	0
Net income (loss)	0	0
Assets:
Total loans outstanding	(1,178,809)	(1,116,370)
Deferred loan origination costs	0	0
Loans to members	(1,178,809)	(1,116,370)
Less: Allowance for credit losses	2,982	0
Loans to members, net	(1,175,827)	(1,116,370)
Other assets	(87,970)	(88,987)
Total assets	(1,263,797)	(1,205,357)
Intersegment eliminations | Interest rate swaps
Statement of operations:
Interest expense	0	0
Non-interest income:
Derivative forward value gains (losses)	0	0
CFC | Operating segments
Statement of operations:
Interest income	304,984	281,307
Interest expense	(209,468)	(174,777)
Derivative cash settlements interest expense	(10,528)	(27,145)
Interest expense	(219,996)	(201,922)
Net interest income	84,988	79,385
Provision for credit losses	(3,496)	(4,003)
Net interest income after provision for credit losses	81,492	75,382
Non-interest income:
Fee and other income	5,793	5,323
Derivative gains (losses)	0	0
Investment securities losses	(3,679)	(2,225)
Total non-interest income	2,114	3,098
Non-interest expense:
General and administrative expenses	(25,012)	(23,654)
Other non-interest expense	(322)	(256)
Total non-interest expense	(25,334)	(23,910)
Income (loss) before income taxes	58,272	54,570
Income tax benefit (provision)	0	0
Net income (loss)	58,272	54,570
Assets:
Total loans outstanding	30,656,301	28,849,795
Deferred loan origination costs	12,335	11,875
Loans to members	30,668,636	28,861,670
Less: Allowance for credit losses	(71,056)	(89,535)
Loans to members, net	30,597,580	28,772,135
Other assets	1,456,295	1,288,781
Total assets	32,053,875	30,060,916
CFC | Operating segments | Interest rate swaps
Statement of operations:
Interest expense	0	0
Non-interest income:
Derivative forward value gains (losses)	0	0
NCSC and RTFC | Operating segments
Statement of operations:
Interest income	11,936	10,453
Interest expense	(9,942)	(8,492)
Derivative cash settlements interest expense	(257)	(418)
Interest expense	(10,199)	(8,910)
Net interest income	1,737	1,543
Provision for credit losses	(247)	(206)
Net interest income after provision for credit losses	1,490	1,337
Non-interest income:
Fee and other income	919	884
Derivative gains (losses)	0	0
Investment securities losses	0	0
Total non-interest income	919	884
Non-interest expense:
General and administrative expenses	(2,535)	(2,151)
Other non-interest expense	(381)	(465)
Total non-interest expense	(2,916)	(2,616)
Income (loss) before income taxes	(507)	(395)
Income tax benefit (provision)	(263)	93
Net income (loss)	(770)	(302)
Assets:
Total loans outstanding	1,197,504	1,137,333
Deferred loan origination costs	0	0
Loans to members	1,197,504	1,137,333
Less: Allowance for credit losses	(2,982)	0
Loans to members, net	1,194,522	1,137,333
Other assets	97,694	98,855
Total assets	1,292,216	1,236,188
NCSC and RTFC | Operating segments | Interest rate swaps
Statement of operations:
Interest expense	0	0
Non-interest income:
Derivative forward value gains (losses)	$ 0	$ 0