Business Segments - Segment Results and Total Assets (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Feb. 28, 2022 USD ($)	Feb. 28, 2021 USD ($)	Feb. 28, 2022 USD ($) segment	Feb. 28, 2021 USD ($)	Nov. 30, 2021 USD ($)	May 31, 2021 USD ($)	Nov. 30, 2020 USD ($)	May 31, 2020 USD ($)
Segment Information
Number of operating segments | segment			3
Statement of operations:
Interest income	$ 285,206	$ 278,172	$ 851,626	$ 834,255
Interest expense	(173,654)	(173,040)	(522,027)	(527,438)
Derivative cash settlements interest expense	0	0	0	0
Interest expense	(173,654)	(173,040)	(522,027)	(527,438)
Net interest income	111,552	105,132	329,599	306,817
Benefit for credit losses	12,749	(33,023)	12,146	(34,987)
Net interest income after benefit (provision) for credit losses	124,301	72,109	341,745	271,830
Non-interest income:
Fee and other income	4,270	3,819	13,042	13,667
Derivative forward value gains			122,930	558,266
Derivative gains	169,280	330,196	43,203	471,759
Investment securities losses	(11,621)	(2,807)	(18,190)	491
Total non-interest income	161,929	331,208	38,055	485,917
Non-interest expense:
General and administrative expenses	(23,079)	(23,562)	(70,384)	(70,361)
Losses on early extinguishment of debt	(578)	0	(696)	(1,455)
Other non-interest expense	(265)	(301)	(834)	(956)
Total non-interest expense	(23,922)	(23,863)	(71,914)	(72,772)
Income before income taxes	262,308	379,454	307,886	684,975
Income tax provision	(343)	(507)	(524)	(920)
Net income	261,965	378,947	307,362	684,055
Assets:
Total loans outstanding	29,508,363	28,315,671	29,508,363	28,315,671		$ 28,415,107
Deferred loan origination costs	12,018	11,810	12,018	11,810
Loans to members	29,520,381	28,327,481	29,520,381	28,327,481
Less: Allowance for credit losses	(73,386)	(92,012)	(73,386)	(92,012)	$ (86,135)	(85,532)	$ (58,989)	$ (53,125)
Loans to members, net	29,446,995	28,235,469	29,446,995	28,235,469		28,341,429
Other assets	1,036,527	1,179,519	1,036,527	1,179,519
Total assets	30,483,522	29,414,988	30,483,522	29,414,988		$ 29,638,363
Interest rate swaps
Statement of operations:
Interest expense	(26,212)	(29,735)	(79,727)	(86,507)
Derivative cash settlements interest expense	(26,212)	(29,735)	(79,727)	(86,507)
Non-interest income:
Derivative forward value gains	195,492	359,931	122,930	558,266
Derivative gains	169,280	330,196	43,203	471,759
Operating segments
Statement of operations:
Interest income	293,979	287,054	877,843	861,139
Interest expense	(182,427)	(181,922)	(548,244)	(554,322)
Derivative cash settlements interest expense	(26,212)	(29,735)	(79,727)	(86,507)
Interest expense	(208,639)	(211,657)	(627,971)	(640,829)
Net interest income	85,340	75,397	249,872	220,310
Benefit for credit losses	12,751	(36,611)	15,005	(37,468)
Net interest income after benefit (provision) for credit losses	98,091	38,786	264,877	182,842
Non-interest income:
Fee and other income	6,275	9,367	16,763	21,825
Derivative gains	0	0	0	0
Investment securities losses	(11,621)	(2,807)	(18,190)	491
Total non-interest income	(5,346)	6,560	(1,427)	22,316
Non-interest expense:
General and administrative expenses	(24,674)	(25,083)	(75,170)	(75,068)
Losses on early extinguishment of debt	(578)		(696)	(1,455)
Other non-interest expense	(677)	(740)	(2,628)	(1,926)
Total non-interest expense	(25,929)	(25,823)	(78,494)	(78,449)
Income before income taxes	66,816	19,523	184,956	126,709
Income tax provision	(343)	(507)	(524)	(920)
Net income	66,473	19,016	184,432	125,789
Assets:
Total loans outstanding	30,675,094	29,447,080	30,675,094	29,447,080
Deferred loan origination costs	12,018	11,810	12,018	11,810
Loans to members	30,687,112	29,458,890	30,687,112	29,458,890
Less: Allowance for credit losses	(76,596)	(100,180)	(76,596)	(100,180)
Loans to members, net	30,610,516	29,358,710	30,610,516	29,358,710
Other assets	1,123,550	1,277,543	1,123,550	1,277,543
Total assets	31,734,066	30,636,253	31,734,066	30,636,253
Operating segments | Interest rate swaps
Statement of operations:
Interest expense	0	0	0	0
Non-interest income:
Derivative forward value gains	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	26,212	29,735	79,727	86,507
Interest expense	26,212	29,735	79,727	86,507
Net interest income	26,212	29,735	79,727	86,507
Benefit for credit losses	0	0	0	0
Net interest income after benefit (provision) for credit losses	26,212	29,735	79,727	86,507
Non-interest income:
Fee and other income	0	0	0	0
Derivative gains	169,280	330,196	43,203	471,759
Investment securities losses	0	0	0	0
Total non-interest income	169,280	330,196	43,203	471,759
Non-interest expense:
General and administrative expenses	0	0	0	0
Losses on early extinguishment of debt	0		0	0
Other non-interest expense	0	0	0	0
Total non-interest expense	0	0	0	0
Income before income taxes	195,492	359,931	122,930	558,266
Income tax provision	0	0	0	0
Net income	195,492	359,931	122,930	558,266
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	(26,212)	(29,735)	(79,727)	(86,507)
Non-interest income:
Derivative forward value gains	195,492	359,931	122,930	558,266
Intersegment eliminations
Statement of operations:
Interest income	(8,773)	(8,882)	(26,217)	(26,884)
Interest expense	8,773	8,882	26,217	26,884
Derivative cash settlements interest expense	0	0	0	0
Interest expense	8,773	8,882	26,217	26,884
Net interest income	0	0	0	0
Benefit for credit losses	(2)	3,588	(2,859)	2,481
Net interest income after benefit (provision) for credit losses	(2)	3,588	(2,859)	2,481
Non-interest income:
Fee and other income	(2,005)	(5,548)	(3,721)	(8,158)
Derivative gains	0	0	0	0
Investment securities losses	0	0	0	0
Total non-interest income	(2,005)	(5,548)	(3,721)	(8,158)
Non-interest expense:
General and administrative expenses	1,595	1,521	4,786	4,707
Losses on early extinguishment of debt	0		0	0
Other non-interest expense	412	439	1,794	970
Total non-interest expense	2,007	1,960	6,580	5,677
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	(1,166,731)	(1,131,409)	(1,166,731)	(1,131,409)
Deferred loan origination costs	0	0	0	0
Loans to members	(1,166,731)	(1,131,409)	(1,166,731)	(1,131,409)
Less: Allowance for credit losses	3,210	8,168	3,210	8,168
Loans to members, net	(1,163,521)	(1,123,241)	(1,163,521)	(1,123,241)
Other assets	(87,023)	(98,024)	(87,023)	(98,024)
Total assets	(1,250,544)	(1,221,265)	(1,250,544)	(1,221,265)
Intersegment eliminations | Interest rate swaps
Statement of operations:
Interest expense	0	0	0	0
Non-interest income:
Derivative forward value gains	0	0	0	0
CFC | Operating segments
Statement of operations:
Interest income	283,162	276,153	845,600	828,222
Interest expense	(173,654)	(173,039)	(522,027)	(527,437)
Derivative cash settlements interest expense	(25,802)	(29,307)	(78,480)	(85,240)
Interest expense	(199,456)	(202,346)	(600,507)	(612,677)
Net interest income	83,706	73,807	245,093	215,545
Benefit for credit losses	12,749	(33,023)	12,146	(34,987)
Net interest income after benefit (provision) for credit losses	96,455	40,784	257,239	180,558
Non-interest income:
Fee and other income	5,590	5,059	17,006	17,347
Derivative gains	0	0	0	0
Investment securities losses	(11,621)	(2,807)	(18,190)	491
Total non-interest income	(6,031)	2,252	(1,184)	17,838
Non-interest expense:
General and administrative expenses	(22,690)	(23,174)	(69,060)	(69,124)
Losses on early extinguishment of debt	(578)		(696)	(1,455)
Other non-interest expense	(265)	(301)	(834)	(956)
Total non-interest expense	(23,533)	(23,475)	(70,590)	(71,535)
Income before income taxes	66,891	19,561	185,465	126,861
Income tax provision	0	0	0	0
Net income	66,891	19,561	185,465	126,861
Assets:
Total loans outstanding	29,487,207	28,294,236	29,487,207	28,294,236
Deferred loan origination costs	12,018	11,810	12,018	11,810
Loans to members	29,499,225	28,306,046	29,499,225	28,306,046
Less: Allowance for credit losses	(73,386)	(92,012)	(73,386)	(92,012)
Loans to members, net	29,425,839	28,214,034	29,425,839	28,214,034
Other assets	1,027,433	1,169,521	1,027,433	1,169,521
Total assets	30,453,272	29,383,555	30,453,272	29,383,555
CFC | Operating segments | Interest rate swaps
Statement of operations:
Interest expense	0	0	0	0
Non-interest income:
Derivative forward value gains	0	0	0	0
NCSC and RTFC | Operating segments
Statement of operations:
Interest income	10,817	10,901	32,243	32,917
Interest expense	(8,773)	(8,883)	(26,217)	(26,885)
Derivative cash settlements interest expense	(410)	(428)	(1,247)	(1,267)
Interest expense	(9,183)	(9,311)	(27,464)	(28,152)
Net interest income	1,634	1,590	4,779	4,765
Benefit for credit losses	2	(3,588)	2,859	(2,481)
Net interest income after benefit (provision) for credit losses	1,636	(1,998)	7,638	2,284
Non-interest income:
Fee and other income	685	4,308	(243)	4,478
Derivative gains	0	0	0	0
Investment securities losses	0	0	0	0
Total non-interest income	685	4,308	(243)	4,478
Non-interest expense:
General and administrative expenses	(1,984)	(1,909)	(6,110)	(5,944)
Losses on early extinguishment of debt	0		0	0
Other non-interest expense	(412)	(439)	(1,794)	(970)
Total non-interest expense	(2,396)	(2,348)	(7,904)	(6,914)
Income before income taxes	(75)	(38)	(509)	(152)
Income tax provision	(343)	(507)	(524)	(920)
Net income	(418)	(545)	(1,033)	(1,072)
Assets:
Total loans outstanding	1,187,887	1,152,844	1,187,887	1,152,844
Deferred loan origination costs	0	0	0	0
Loans to members	1,187,887	1,152,844	1,187,887	1,152,844
Less: Allowance for credit losses	(3,210)	(8,168)	(3,210)	(8,168)
Loans to members, net	1,184,677	1,144,676	1,184,677	1,144,676
Other assets	96,117	108,022	96,117	108,022
Total assets	1,280,794	1,252,698	1,280,794	1,252,698
NCSC and RTFC | Operating segments | Interest rate swaps
Statement of operations:
Interest expense	0	0	0	0
Non-interest income:
Derivative forward value gains	$ 0	$ 0	$ 0	$ 0