Business Segments - Segment Results and Total Assets (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Nov. 30, 2021 USD ($)	Nov. 30, 2020 USD ($)	Nov. 30, 2021 USD ($) segment	Nov. 30, 2020 USD ($)	Aug. 31, 2021 USD ($)	May 31, 2021 USD ($)	Aug. 31, 2020 USD ($)	May 31, 2020 USD ($)
Segment Information
Number of operating segments | segment			3
Statement of operations:
Interest income	$ 283,152	$ 276,499	$ 566,420	$ 556,083
Interest expense	(173,596)	(174,422)	(348,373)	(354,398)
Derivative cash settlements interest expense	0	0	0	0
Interest expense	(173,596)	(174,422)	(348,373)	(354,398)
Net interest income	109,556	102,077	218,047	201,685
Benefit (provision) for credit losses	3,400	(1,638)	(603)	(1,964)
Net interest income after benefit (provision) for credit losses	112,956	100,439	217,444	199,721
Non-interest income:
Fee and other income	4,831	6,332	8,772	9,848
Derivative forward value gains (losses)			(72,562)	198,335
Derivative gains (losses)	46,086	81,287	(126,077)	141,563
Investment securities losses	(4,344)	(1,361)	(6,569)	3,298
Total non-interest income	46,573	86,258	(123,874)	154,709
Non-interest expense:
General and administrative expenses	(23,095)	(24,136)	(47,305)	(46,799)
Losses on early extinguishment of debt	(118)	(1,455)	(118)	(1,455)
Other non-interest expense	(313)	(323)	(569)	(655)
Total non-interest expense	(23,526)	(25,914)	(47,992)	(48,909)
Income before income taxes	136,003	160,783	45,578	305,521
Income tax provision	(274)	(262)	(181)	(413)
Net income	135,729	160,521	45,397	305,108
Assets:
Total loans outstanding	28,934,814	27,051,163	28,934,814	27,051,163		$ 28,415,107
Deferred loan origination costs	12,056	11,806	12,056	11,806
Loans to members	28,946,870	27,062,969	28,946,870	27,062,969
Less: Allowance for credit losses	(86,135)	(58,989)	(86,135)	(58,989)	$ (89,535)	(85,532)	$ (57,351)	$ (53,125)
Loans to members, net	28,860,735	27,003,980	28,860,735	27,003,980		28,341,429
Other assets	1,154,844	1,172,122	1,154,844	1,172,122
Total assets	30,015,579	28,176,102	30,015,579	28,176,102		$ 29,638,363
Interest rate swaps
Statement of operations:
Interest expense	(25,952)	(29,800)	(53,515)	(56,772)
Derivative cash settlements interest expense	(25,952)	(29,800)	(53,515)	(56,772)
Non-interest income:
Derivative forward value gains (losses)	72,038	111,087	(72,562)	198,335
Derivative gains (losses)	46,086	81,287	(126,077)	141,563
Operating segments
Statement of operations:
Interest income	292,104	285,481	583,864	574,085
Interest expense	(182,548)	(183,404)	(365,817)	(372,400)
Derivative cash settlements interest expense	(25,952)	(29,800)	(53,515)	(56,772)
Interest expense	(208,500)	(213,204)	(419,332)	(429,172)
Net interest income	83,604	72,277	164,532	144,913
Benefit (provision) for credit losses	6,463	(1,597)	2,254	(857)
Net interest income after benefit (provision) for credit losses	90,067	70,680	166,786	144,056
Non-interest income:
Fee and other income	4,281	8,199	10,488	12,458
Derivative gains (losses)	0	0	0	0
Investment securities losses	(4,344)	(1,361)	(6,569)	3,298
Total non-interest income	(63)	6,838	3,919	15,756
Non-interest expense:
General and administrative expenses	(24,691)	(25,728)	(50,496)	(49,985)
Losses on early extinguishment of debt	(118)	(1,455)	(118)	(1,455)
Other non-interest expense	(1,230)	(639)	(1,951)	(1,186)
Total non-interest expense	(26,039)	(27,822)	(52,565)	(52,626)
Income before income taxes	63,965	49,696	118,140	107,186
Income tax provision	(274)	(262)	(181)	(413)
Net income	63,691	49,434	117,959	106,773
Assets:
Total loans outstanding	30,064,831	28,163,389	30,064,831	28,163,389
Deferred loan origination costs	12,056	11,806	12,056	11,806
Loans to members	30,076,887	28,175,195	30,076,887	28,175,195
Less: Allowance for credit losses	(89,347)	(63,569)	(89,347)	(63,569)
Loans to members, net	29,987,540	28,111,626	29,987,540	28,111,626
Other assets	1,244,056	1,269,235	1,244,056	1,269,235
Total assets	31,231,596	29,380,861	31,231,596	29,380,861
Operating segments | Interest rate swaps
Statement of operations:
Interest expense	0	0	0	0
Non-interest income:
Derivative forward value gains (losses)	0	0	0	0
Reclasses and adjustments
Statement of operations:
Interest income	0	0	0	0
Interest expense	0	0	0	0
Derivative cash settlements interest expense	25,952	29,800	53,515	56,772
Interest expense	25,952	29,800	53,515	56,772
Net interest income	25,952	29,800	53,515	56,772
Benefit (provision) for credit losses	0	0	0	0
Net interest income after benefit (provision) for credit losses	25,952	29,800	53,515	56,772
Non-interest income:
Fee and other income	0	0	0	0
Derivative gains (losses)	46,086	81,287	(126,077)	141,563
Investment securities losses	0	0	0	0
Total non-interest income	46,086	81,287	(126,077)	141,563
Non-interest expense:
General and administrative expenses	0	0	0	0
Losses on early extinguishment of debt	0	0	0	0
Other non-interest expense	0	0	0	0
Total non-interest expense	0	0	0	0
Income before income taxes	72,038	111,087	(72,562)	198,335
Income tax provision	0	0	0	0
Net income	72,038	111,087	(72,562)	198,335
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
Statement of operations:
Interest expense	(25,952)	(29,800)	(53,515)	(56,772)
Non-interest income:
Derivative forward value gains (losses)	72,038	111,087	(72,562)	198,335
Intersegment eliminations
Statement of operations:
Interest income	(8,952)	(8,982)	(17,444)	(18,002)
Interest expense	8,952	8,982	17,444	18,002
Derivative cash settlements interest expense	0	0	0	0
Interest expense	8,952	8,982	17,444	18,002
Net interest income	0	0	0	0
Benefit (provision) for credit losses	(3,063)	(41)	(2,857)	(1,107)
Net interest income after benefit (provision) for credit losses	(3,063)	(41)	(2,857)	(1,107)
Non-interest income:
Fee and other income	550	(1,867)	(1,716)	(2,610)
Derivative gains (losses)	0	0	0	0
Investment securities losses	0	0	0	0
Total non-interest income	550	(1,867)	(1,716)	(2,610)
Non-interest expense:
General and administrative expenses	1,596	1,592	3,191	3,186
Losses on early extinguishment of debt	0	0	0	0
Other non-interest expense	917	316	1,382	531
Total non-interest expense	2,513	1,908	4,573	3,717
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	(1,130,017)	(1,112,226)	(1,130,017)	(1,112,226)
Deferred loan origination costs	0	0	0	0
Loans to members	(1,130,017)	(1,112,226)	(1,130,017)	(1,112,226)
Less: Allowance for credit losses	3,212	4,580	3,212	4,580
Loans to members, net	(1,126,805)	(1,107,646)	(1,126,805)	(1,107,646)
Other assets	(89,212)	(97,113)	(89,212)	(97,113)
Total assets	(1,216,017)	(1,204,759)	(1,216,017)	(1,204,759)
Intersegment eliminations | Interest rate swaps
Statement of operations:
Interest expense	0	0	0	0
Non-interest income:
Derivative forward value gains (losses)	0	0	0	0
CFC | Operating segments
Statement of operations:
Interest income	281,131	274,473	562,438	552,069
Interest expense	(173,596)	(174,422)	(348,373)	(354,398)
Derivative cash settlements interest expense	(25,533)	(29,370)	(52,678)	(55,933)
Interest expense	(199,129)	(203,792)	(401,051)	(410,331)
Net interest income	82,002	70,681	161,387	141,738
Benefit (provision) for credit losses	3,400	(1,638)	(603)	(1,964)
Net interest income after benefit (provision) for credit losses	85,402	69,043	160,784	139,774
Non-interest income:
Fee and other income	6,093	7,513	11,416	12,288
Derivative gains (losses)	0	0	0	0
Investment securities losses	(4,344)	(1,361)	(6,569)	3,298
Total non-interest income	1,749	6,152	4,847	15,586
Non-interest expense:
General and administrative expenses	(22,716)	(23,750)	(46,370)	(45,950)
Losses on early extinguishment of debt	(118)	(1,455)	(118)	(1,455)
Other non-interest expense	(313)	(323)	(569)	(655)
Total non-interest expense	(23,147)	(25,528)	(47,057)	(48,060)
Income before income taxes	64,004	49,667	118,574	107,300
Income tax provision	0	0	0	0
Net income	64,004	49,667	118,574	107,300
Assets:
Total loans outstanding	28,911,891	27,027,010	28,911,891	27,027,010
Deferred loan origination costs	12,056	11,806	12,056	11,806
Loans to members	28,923,947	27,038,816	28,923,947	27,038,816
Less: Allowance for credit losses	(86,135)	(58,989)	(86,135)	(58,989)
Loans to members, net	28,837,812	26,979,827	28,837,812	26,979,827
Other assets	1,144,912	1,161,963	1,144,912	1,161,963
Total assets	29,982,724	28,141,790	29,982,724	28,141,790
CFC | Operating segments | Interest rate swaps
Statement of operations:
Interest expense	0	0	0	0
Non-interest income:
Derivative forward value gains (losses)	0	0	0	0
NCSC and RTFC | Operating segments
Statement of operations:
Interest income	10,973	11,008	21,426	22,016
Interest expense	(8,952)	(8,982)	(17,444)	(18,002)
Derivative cash settlements interest expense	(419)	(430)	(837)	(839)
Interest expense	(9,371)	(9,412)	(18,281)	(18,841)
Net interest income	1,602	1,596	3,145	3,175
Benefit (provision) for credit losses	3,063	41	2,857	1,107
Net interest income after benefit (provision) for credit losses	4,665	1,637	6,002	4,282
Non-interest income:
Fee and other income	(1,812)	686	(928)	170
Derivative gains (losses)	0	0	0	0
Investment securities losses	0	0	0	0
Total non-interest income	(1,812)	686	(928)	170
Non-interest expense:
General and administrative expenses	(1,975)	(1,978)	(4,126)	(4,035)
Losses on early extinguishment of debt	0	0	0	0
Other non-interest expense	(917)	(316)	(1,382)	(531)
Total non-interest expense	(2,892)	(2,294)	(5,508)	(4,566)
Income before income taxes	(39)	29	(434)	(114)
Income tax provision	(274)	(262)	(181)	(413)
Net income	(313)	(233)	(615)	(527)
Assets:
Total loans outstanding	1,152,940	1,136,379	1,152,940	1,136,379
Deferred loan origination costs	0	0	0	0
Loans to members	1,152,940	1,136,379	1,152,940	1,136,379
Less: Allowance for credit losses	(3,212)	(4,580)	(3,212)	(4,580)
Loans to members, net	1,149,728	1,131,799	1,149,728	1,131,799
Other assets	99,144	107,272	99,144	107,272
Total assets	1,248,872	1,239,071	1,248,872	1,239,071
NCSC and RTFC | Operating segments | Interest rate swaps
Statement of operations:
Interest expense	0	0	0	0
Non-interest income:
Derivative forward value gains (losses)	$ 0	$ 0	$ 0	$ 0