Business Segments - Schedule of segment presentation for the consolidated statements of operations and consolidated balance sheets (Details) - USD ($) $ in Thousands	12 Months Ended
	May 31, 2026	May 31, 2025	May 31, 2024	May 31, 2023
Statement of operations:
Interest income	$ 1,796,660	$ 1,703,233	$ 1,593,351
Interest expense	(1,494,894)	(1,442,279)	(1,339,088)
Derivative cash settlements interest income (expense)	0	0	0
Interest income (expense)	(1,494,894)	(1,442,279)	(1,339,088)
Net interest income	301,766	260,954	254,263
Benefit for credit losses	10,362	8,111	5,516
Net interest income after benefit for credit losses	312,128	269,065	259,779
Non-interest income:
Fee and other income	28,719	23,597	22,792
Derivative gains:
Derivative forward value gains (losses)	18,213	(105,070)	264,871
Derivative gains (losses)	82,166	(5,851)	392,037
Investment securities gains (losses)	1,128	5,674	10,772
Total non-interest income	112,013	23,420	425,601
Non-interest expense:
Salaries and employee benefits	(80,083)	(72,171)	(67,401)
Depreciation and amortization	(13,966)	(12,615)	(10,469)
Other non-interest expense	(67,022)	(67,497)	(51,690)
Total non-interest expense	(161,071)	(152,283)	(129,560)
Income before income taxes	263,070	140,202	555,820
Income tax provision	(334)	(188)	(1,504)
Net income	262,736	140,014	554,316
Assets:
Total loans outstanding	38,403,415	37,063,548
Deferred loan origination costs	18,713	16,430
Loans to members	38,422,128	37,079,978
Less: Allowance for credit losses	(29,947)	(40,615)	(48,726)	$ (53,094)
Loans to members, net	38,392,181	37,039,363
Other assets	1,312,166	1,285,686
Total assets	39,704,347	38,325,049
Interest rate swaps
Derivative gains:
Derivative cash settlements interest income	63,953	99,219	127,166
Derivative forward value gains (losses)	18,213	(105,070)	264,871
Derivative gains (losses)	82,166	(5,851)	392,037
Operating segments
Statement of operations:
Interest income	1,879,021	1,781,217	1,666,175
Interest expense	(1,577,255)	(1,520,263)	(1,411,912)
Derivative cash settlements interest income (expense)	63,953	99,219	127,166
Interest income (expense)	(1,513,302)	(1,421,044)	(1,284,746)
Net interest income	365,719	360,173	381,429
Benefit for credit losses	10,257	8,525	3,186
Net interest income after benefit for credit losses	375,976	368,698	384,615
Non-interest income:
Fee and other income	41,137	32,869	35,305
Derivative gains:
Derivative gains (losses)	0	0	0
Investment securities gains (losses)	1,128	5,674	10,772
Total non-interest income	42,265	38,543	46,077
Non-interest expense:
Salaries and employee benefits	(80,083)	(72,171)	(67,401)
Depreciation and amortization	(13,966)	(12,615)	(10,469)
Other non-interest expense	(79,335)	(77,183)	(61,873)
Total non-interest expense	(173,384)	(161,969)	(139,743)
Income before income taxes	244,857	245,272	290,949
Income tax provision	(334)	(188)	(1,504)
Net income	244,523	245,084	289,445
Assets:
Total loans outstanding	40,045,448	38,703,920
Deferred loan origination costs	18,713	16,430
Loans to members	40,064,161	38,720,350
Less: Allowance for credit losses	(35,638)	(46,201)
Loans to members, net	40,028,523	38,674,149
Other assets	1,331,128	1,303,643
Total assets	41,359,651	39,977,792
Operating segments | Interest rate swaps
Derivative gains:
Derivative cash settlements interest income	0	0	0
Derivative forward value gains (losses)	0	0	0
Reclasses and adjustments
Statement of operations:
Interest income	0	0	0
Interest expense	0	0	0
Derivative cash settlements interest income (expense)	(63,953)	(99,219)	(127,166)
Interest income (expense)	(63,953)	(99,219)	(127,166)
Net interest income	(63,953)	(99,219)	(127,166)
Benefit for credit losses	0	0	0
Net interest income after benefit for credit losses	(63,953)	(99,219)	(127,166)
Non-interest income:
Fee and other income	0	0	0
Derivative gains:
Derivative gains (losses)	82,166	(5,851)	392,037
Investment securities gains (losses)	0	0	0
Total non-interest income	82,166	(5,851)	392,037
Non-interest expense:
Salaries and employee benefits	0	0	0
Depreciation and amortization	0	0	0
Other non-interest expense	0	0	0
Total non-interest expense	0	0	0
Income before income taxes	18,213	(105,070)	264,871
Income tax provision	0	0	0
Net income	18,213	(105,070)	264,871
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
Derivative gains:
Derivative cash settlements interest income	63,953	99,219	127,166
Derivative forward value gains (losses)	18,213	(105,070)	264,871
Intersegment eliminations
Statement of operations:
Interest income	(82,361)	(77,984)	(72,824)
Interest expense	82,361	77,984	72,824
Derivative cash settlements interest income (expense)	0	0	0
Interest income (expense)	82,361	77,984	72,824
Net interest income	0	0	0
Benefit for credit losses	105	(414)	2,330
Net interest income after benefit for credit losses	105	(414)	2,330
Non-interest income:
Fee and other income	(12,418)	(9,272)	(12,513)
Derivative gains:
Derivative gains (losses)	0	0	0
Investment securities gains (losses)	0	0	0
Total non-interest income	(12,418)	(9,272)	(12,513)
Non-interest expense:
Salaries and employee benefits	0	0	0
Depreciation and amortization	0	0	0
Other non-interest expense	12,313	9,686	10,183
Total non-interest expense	12,313	9,686	10,183
Income before income taxes	0	0	0
Income tax provision	0	0	0
Net income	0	0	0
Assets:
Total loans outstanding	(1,642,033)	(1,640,372)
Deferred loan origination costs	0	0
Loans to members	(1,642,033)	(1,640,372)
Less: Allowance for credit losses	5,691	5,586
Loans to members, net	(1,636,342)	(1,634,786)
Other assets	(18,962)	(17,957)
Total assets	(1,655,304)	(1,652,743)
Intersegment eliminations | Interest rate swaps
Derivative gains:
Derivative cash settlements interest income	0	0	0
Derivative forward value gains (losses)	0	0	0
CFC | Operating segments
Statement of operations:
Interest income	1,785,859	1,693,507	1,584,071
Interest expense	(1,494,526)	(1,442,027)	(1,339,003)
Derivative cash settlements interest income (expense)	64,178	99,237	127,017
Interest income (expense)	(1,430,348)	(1,342,790)	(1,211,986)
Net interest income	355,511	350,717	372,085
Benefit for credit losses	10,362	8,111	5,516
Net interest income after benefit for credit losses	365,873	358,828	377,601
Non-interest income:
Fee and other income	34,814	29,162	27,857
Derivative gains:
Derivative gains (losses)	0	0	0
Investment securities gains (losses)	1,128	5,674	10,772
Total non-interest income	35,942	34,836	38,629
Non-interest expense:
Salaries and employee benefits	(79,813)	(71,920)	(66,382)
Depreciation and amortization	(13,966)	(12,615)	(10,469)
Other non-interest expense	(63,680)	(64,844)	(49,622)
Total non-interest expense	(157,459)	(149,379)	(126,473)
Income before income taxes	244,356	244,285	289,757
Income tax provision	0	0	0
Net income	244,356	244,285	289,757
Assets:
Total loans outstanding	38,389,050	37,049,692
Deferred loan origination costs	18,713	16,430
Loans to members	38,407,763	37,066,122
Less: Allowance for credit losses	(29,947)	(40,615)
Loans to members, net	38,377,816	37,025,507
Other assets	1,296,147	1,269,575
Total assets	39,673,963	38,295,082
CFC | Operating segments | Interest rate swaps
Derivative gains:
Derivative cash settlements interest income	0	0	0
Derivative forward value gains (losses)	0	0	0
NCSC | Operating segments
Statement of operations:
Interest income	93,162	87,710	82,104
Interest expense	(82,729)	(78,236)	(72,909)
Derivative cash settlements interest income (expense)	(225)	(18)	149
Interest income (expense)	(82,954)	(78,254)	(72,760)
Net interest income	10,208	9,456	9,344
Benefit for credit losses	(105)	414	(2,330)
Net interest income after benefit for credit losses	10,103	9,870	7,014
Non-interest income:
Fee and other income	6,323	3,707	7,448
Derivative gains:
Derivative gains (losses)	0	0	0
Investment securities gains (losses)	0	0	0
Total non-interest income	6,323	3,707	7,448
Non-interest expense:
Salaries and employee benefits	(270)	(251)	(1,019)
Depreciation and amortization	0	0	0
Other non-interest expense	(15,655)	(12,339)	(12,251)
Total non-interest expense	(15,925)	(12,590)	(13,270)
Income before income taxes	501	987	1,192
Income tax provision	(334)	(188)	(1,504)
Net income	167	799	(312)
Assets:
Total loans outstanding	1,656,398	1,654,228
Deferred loan origination costs	0	0
Loans to members	1,656,398	1,654,228
Less: Allowance for credit losses	(5,691)	(5,586)
Loans to members, net	1,650,707	1,648,642
Other assets	34,981	34,068
Total assets	1,685,688	1,682,710
NCSC | Operating segments | Interest rate swaps
Derivative gains:
Derivative cash settlements interest income	0	0	0
Derivative forward value gains (losses)	$ 0	$ 0	$ 0