Business Segments - Schedule of Segment Presentation for the Consolidated Statements of Operations and Consolidated Balance Sheets (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Feb. 28, 2026	Feb. 28, 2025	Feb. 28, 2026	Feb. 28, 2025	Nov. 30, 2025	May 31, 2025	Nov. 30, 2024	May 31, 2024
Results of operations:
Interest income	$ 449,570	$ 428,860	$ 1,342,690	$ 1,266,856
Interest expense	(370,010)	(361,918)	(1,120,145)	(1,072,821)
Derivative cash settlements interest income (expense)	0	0	0	0
Interest expense	(370,010)	(361,918)	(1,120,145)	(1,072,821)
Net interest income	79,560	66,942	222,545	194,035
Benefit (provision) for credit losses	7,405	6,093	4,597	4,270
Net interest income after benefit for credit losses	86,965	73,035	227,142	198,305
Non-interest income (loss):
Fee and other income	6,796	5,982	20,558	17,074
Derivative losses:
Derivative forward value gains (losses)			(133,659)	(127,921)
Derivative gains (losses)	(32,370)	40,142	(83,215)	(49,145)
Investment securities gains	(142)	663	276	6,891
Total non-interest income (loss)	(25,716)	46,787	(62,381)	(25,180)
Non-interest expense:
Salaries and employee benefits	(20,036)	(18,123)	(59,274)	(52,750)
Consulting	(3,183)	(2,944)	(8,162)	(10,537)
Depreciation and amortization	(3,362)	(3,192)	(10,098)	(9,352)
Other non-interest expense	(11,769)	(9,361)	(40,102)	(33,765)
Total non-interest expense	(38,350)	(33,620)	(117,636)	(106,404)
Income before income taxes	22,899	86,202	47,125	66,721
Income tax provision	(162)	(66)	(148)	(108)
Net income	22,737	86,136	46,977	66,613
Assets:
Total loans outstanding	38,731,679	36,463,440	38,731,679	36,463,440		$ 37,063,548
Deferred loan origination costs	18,203	15,769	18,203	15,769
Loans to members	38,749,882	36,479,209	38,749,882	36,479,209		37,079,978
Less: Allowance for credit losses	(35,712)	(44,456)	(35,712)	(44,456)	$ (43,423)	(40,615)	$ (50,549)	$ (48,726)
Loans to members, net	38,714,170	36,434,753	38,714,170	36,434,753		37,039,363
Other assets	1,178,763	1,343,990	1,178,763	1,343,990
Total assets	39,892,933	37,778,743	39,892,933	37,778,743		$ 38,325,049
Interest rate swaps
Results of operations:
Derivative cash settlements interest income (expense)	12,592	20,309	50,444	78,776
Derivative losses:
Derivative cash settlements interest income	12,592	20,309	50,444	78,776
Derivative forward value gains (losses)	(44,962)	19,833	(133,659)	(127,921)
Derivative gains (losses)	(32,370)	40,142	(83,215)	(49,145)
Operating Segments
Results of operations:
Interest income	470,549	448,153	1,404,477	1,325,276
Interest expense	(390,989)	(381,211)	(1,181,932)	(1,131,241)
Derivative cash settlements interest income (expense)	12,592	20,309	50,444	78,776
Interest expense	(378,397)	(360,902)	(1,131,488)	(1,052,465)
Net interest income	92,152	87,251	272,989	272,811
Benefit (provision) for credit losses	6,768	5,817	2,931	3,477
Net interest income after benefit for credit losses	98,920	93,068	275,920	276,288
Non-interest income (loss):
Fee and other income	10,184	8,588	30,944	24,890
Derivative losses:
Derivative gains (losses)	0	0	0	0
Investment securities gains	(142)	663	276	6,891
Total non-interest income (loss)	10,042	9,251	31,220	31,781
Non-interest expense:
Salaries and employee benefits	(20,036)	(18,123)	(59,274)	(52,750)
Consulting	(3,183)	(2,944)	(8,162)	(10,537)
Depreciation and amortization	(3,362)	(3,192)	(10,098)	(9,352)
Other non-interest expense	(14,520)	(11,691)	(48,822)	(40,788)
Total non-interest expense	(41,101)	(35,950)	(126,356)	(113,427)
Income before income taxes	67,861	66,369	180,784	194,642
Income tax provision	(162)	(66)	(148)	(108)
Net income	67,699	66,303	180,636	194,534
Assets:
Total loans outstanding	40,499,602	38,058,480	40,499,602	38,058,480
Deferred loan origination costs	18,203	15,769	18,203	15,769
Loans to members	40,517,805	38,074,249	40,517,805	38,074,249
Less: Allowance for credit losses	(42,964)	(51,249)	(42,964)	(51,249)
Loans to members, net	40,474,841	38,023,000	40,474,841	38,023,000
Other assets	1,195,775	1,357,665	1,195,775	1,357,665
Total assets	41,670,616	39,380,665	41,670,616	39,380,665
Operating Segments | Interest rate swaps
Derivative losses:
Derivative cash settlements interest income	0	0	0	0
Derivative forward value gains (losses)	0	0	0	0
Reclasses and Adjustments
Results of operations:
Interest income	0	0	0	0
Interest expense	0	0	0	0
Derivative cash settlements interest income (expense)	(12,592)	(20,309)	(50,444)	(78,776)
Interest expense	(12,592)	(20,309)	(50,444)	(78,776)
Net interest income	(12,592)	(20,309)	(50,444)	(78,776)
Benefit (provision) for credit losses	0	0	0	0
Net interest income after benefit for credit losses	(12,592)	(20,309)	(50,444)	(78,776)
Non-interest income (loss):
Fee and other income	0	0	0	0
Derivative losses:
Derivative gains (losses)	(32,370)	40,142	(83,215)	(49,145)
Investment securities gains	0	0	0	0
Total non-interest income (loss)	(32,370)	40,142	(83,215)	(49,145)
Non-interest expense:
Salaries and employee benefits	0	0	0	0
Consulting	0	0	0	0
Depreciation and amortization	0	0	0	0
Other non-interest expense	0	0	0	0
Total non-interest expense	0	0	0	0
Income before income taxes	(44,962)	19,833	(133,659)	(127,921)
Income tax provision	0	0	0	0
Net income	(44,962)	19,833	(133,659)	(127,921)
Assets:
Total loans outstanding	0	0	0	0
Deferred loan origination costs	0	0	0	0
Loans to members	0	0	0	0
Less: Allowance for credit losses	0	0	0	0
Loans to members, net	0	0	0	0
Other assets	0	0	0	0
Total assets	0	0	0	0
Reclasses and Adjustments | Interest rate swaps
Derivative losses:
Derivative cash settlements interest income	12,592	20,309	50,444	78,776
Derivative forward value gains (losses)	(44,962)	19,833	(133,659)	(127,921)
Intersegment Eliminations
Results of operations:
Interest income	(20,979)	(19,293)	(61,787)	(58,420)
Interest expense	20,979	19,293	61,787	58,420
Derivative cash settlements interest income (expense)	0	0	0	0
Interest expense	20,979	19,293	61,787	58,420
Net interest income	0	0	0	0
Benefit (provision) for credit losses	637	276	1,666	793
Net interest income after benefit for credit losses	637	276	1,666	793
Non-interest income (loss):
Fee and other income	(3,388)	(2,606)	(10,386)	(7,816)
Derivative losses:
Derivative gains (losses)	0	0	0	0
Investment securities gains	0	0	0	0
Total non-interest income (loss)	(3,388)	(2,606)	(10,386)	(7,816)
Non-interest expense:
Salaries and employee benefits	0	0	0	0
Consulting	0	0	0	0
Depreciation and amortization	0	0	0	0
Other non-interest expense	2,751	2,330	8,720	7,023
Total non-interest expense	2,751	2,330	8,720	7,023
Income before income taxes	0	0	0	0
Income tax provision	0	0	0	0
Net income	0	0	0	0
Assets:
Total loans outstanding	(1,767,923)	(1,595,040)	(1,767,923)	(1,595,040)
Deferred loan origination costs	0	0	0	0
Loans to members	(1,767,923)	(1,595,040)	(1,767,923)	(1,595,040)
Less: Allowance for credit losses	7,252	6,793	7,252	6,793
Loans to members, net	(1,760,671)	(1,588,247)	(1,760,671)	(1,588,247)
Other assets	(17,012)	(13,675)	(17,012)	(13,675)
Total assets	(1,777,683)	(1,601,922)	(1,777,683)	(1,601,922)
Intersegment Eliminations | Interest rate swaps
Derivative losses:
Derivative cash settlements interest income	0	0	0	0
Derivative forward value gains (losses)	0	0	0	0
CFC | Operating Segments
Results of operations:
Interest income	446,767	426,442	1,334,660	1,259,727
Interest expense	(369,919)	(361,847)	(1,119,870)	(1,072,648)
Derivative cash settlements interest income (expense)	12,668	20,343	50,593	78,760
Interest expense	(357,251)	(341,504)	(1,069,277)	(993,888)
Net interest income	89,516	84,938	265,383	265,839
Benefit (provision) for credit losses	7,405	6,093	4,597	4,270
Net interest income after benefit for credit losses	96,921	91,031	269,980	270,109
Non-interest income (loss):
Fee and other income	8,241	7,241	24,859	21,335
Derivative losses:
Derivative gains (losses)	0	0	0	0
Investment securities gains	(142)	663	276	6,891
Total non-interest income (loss)	8,099	7,904	25,135	28,226
Non-interest expense:
Salaries and employee benefits	(19,970)	(18,060)	(59,073)	(52,565)
Consulting	(3,139)	(2,887)	(8,033)	(10,341)
Depreciation and amortization	(3,362)	(3,192)	(10,098)	(9,352)
Other non-interest expense	(11,189)	(8,613)	(37,628)	(32,025)
Total non-interest expense	(37,660)	(32,752)	(114,832)	(104,283)
Income before income taxes	67,360	66,183	180,283	194,052
Income tax provision	0	0	0	0
Net income	67,360	66,183	180,283	194,052
Assets:
Total loans outstanding	38,717,223	36,449,424	38,717,223	36,449,424
Deferred loan origination costs	18,203	15,769	18,203	15,769
Loans to members	38,735,426	36,465,193	38,735,426	36,465,193
Less: Allowance for credit losses	(35,712)	(44,456)	(35,712)	(44,456)
Loans to members, net	38,699,714	36,420,737	38,699,714	36,420,737
Other assets	1,162,459	1,330,000	1,162,459	1,330,000
Total assets	39,862,173	37,750,737	39,862,173	37,750,737
CFC | Operating Segments | Interest rate swaps
Derivative losses:
Derivative cash settlements interest income	0	0	0	0
Derivative forward value gains (losses)	0	0	0	0
NCSC | Operating Segments
Results of operations:
Interest income	23,782	21,711	69,817	65,549
Interest expense	(21,070)	(19,364)	(62,062)	(58,593)
Derivative cash settlements interest income (expense)	(76)	(34)	(149)	16
Interest expense	(21,146)	(19,398)	(62,211)	(58,577)
Net interest income	2,636	2,313	7,606	6,972
Benefit (provision) for credit losses	(637)	(276)	(1,666)	(793)
Net interest income after benefit for credit losses	1,999	2,037	5,940	6,179
Non-interest income (loss):
Fee and other income	1,943	1,347	6,085	3,555
Derivative losses:
Derivative gains (losses)	0	0	0	0
Investment securities gains	0	0	0	0
Total non-interest income (loss)	1,943	1,347	6,085	3,555
Non-interest expense:
Salaries and employee benefits	(66)	(63)	(201)	(185)
Consulting	(44)	(57)	(129)	(196)
Depreciation and amortization	0	0	0	0
Other non-interest expense	(3,331)	(3,078)	(11,194)	(8,763)
Total non-interest expense	(3,441)	(3,198)	(11,524)	(9,144)
Income before income taxes	501	186	501	590
Income tax provision	(162)	(66)	(148)	(108)
Net income	339	120	353	482
Assets:
Total loans outstanding	1,782,379	1,609,056	1,782,379	1,609,056
Deferred loan origination costs	0	0	0	0
Loans to members	1,782,379	1,609,056	1,782,379	1,609,056
Less: Allowance for credit losses	(7,252)	(6,793)	(7,252)	(6,793)
Loans to members, net	1,775,127	1,602,263	1,775,127	1,602,263
Other assets	33,316	27,665	33,316	27,665
Total assets	1,808,443	1,629,928	1,808,443	1,629,928
NCSC | Operating Segments | Interest rate swaps
Derivative losses:
Derivative cash settlements interest income	0	0	0	0
Derivative forward value gains (losses)	$ 0	$ 0	$ 0	$ 0