Business Segments - Segment Results and Total Assets (Details) $ in Thousands	3 Months Ended
	Aug. 31, 2023 USD ($) segment	Aug. 31, 2022 USD ($)	May 31, 2023 USD ($)	May 31, 2022 USD ($)
Segment Information
Number of operating segments | segment	3
Statement of operations:
Interest income	$ 380,956	$ 306,978
Interest expense	(316,281)	(209,468)
Derivative cash settlements interest income (expense)	0	0
Interest expense	(316,281)	(209,468)
Net interest income	64,675	97,510
Provision for credit losses	(800)	(3,496)
Net interest income after provision for credit losses	63,875	94,014
Non-interest income:
Fee and other income	4,537	4,056
Derivative gains:
Derivative forward value gains	162,018	104,372
Derivative gains	189,887	93,587
Investment securities gains	2,933	(3,679)
Total non-interest income	197,357	93,964
Non-interest expense:
General and administrative expenses	(31,503)	(25,519)
Losses on early extinguishment of debt	(939)	0
Other non-interest expense	(178)	(322)
Total non-interest expense	(32,620)	(25,841)
Income before income taxes	228,612	162,137
Income tax provision	(328)	(263)
Net income (loss)	228,284	161,874
Assets:
Total loans outstanding	33,083,493	30,674,996	$ 32,519,349
Deferred loan origination costs	13,153	12,335
Loans to members	33,096,646	30,687,331	32,532,086
Less: Allowance for credit losses	(54,926)	(71,056)	(53,094)	$ (67,560)
Loans to members, net	33,041,720	30,616,275	32,478,992
Other assets	1,652,443	1,466,019
Total assets	34,694,163	32,082,294	$ 34,012,060
Interest rate swaps
Statement of operations:
Derivative cash settlements interest income (expense)	27,869	(10,785)
Derivative gains:
Derivative cash settlements interest income	27,869	(10,785)
Derivative forward value gains	162,018	104,372
Derivative gains	189,887	93,587
Operating segments
Statement of operations:
Interest income	397,662	316,920
Interest expense	(332,987)	(219,410)
Derivative cash settlements interest income (expense)	27,869	(10,785)
Interest expense	(305,118)	(230,195)
Net interest income	92,544	86,725
Provision for credit losses	(1,702)	(3,743)
Net interest income after provision for credit losses	90,842	82,982
Non-interest income:
Fee and other income	7,993	6,712
Derivative gains:
Derivative gains	0	0
Investment securities gains	2,933	(3,679)
Total non-interest income	10,926	3,033
Non-interest expense:
General and administrative expenses	(33,699)	(27,547)
Losses on early extinguishment of debt	(939)
Other non-interest expense	(536)	(703)
Total non-interest expense	(35,174)	(28,250)
Income before income taxes	66,594	57,765
Income tax provision	(328)	(263)
Net income (loss)	66,266	57,502
Assets:
Total loans outstanding	34,525,708	31,853,805
Deferred loan origination costs	13,153	12,335
Loans to members	34,538,861	31,866,140
Less: Allowance for credit losses	(59,498)	(74,038)
Loans to members, net	34,479,363	31,792,102
Other assets	1,720,747	1,553,989
Total assets	36,200,110	33,346,091
Operating segments | Interest rate swaps
Derivative gains:
Derivative cash settlements interest income	0	0
Derivative forward value gains	0	0
Reclasses and adjustments
Statement of operations:
Interest income	0	0
Interest expense	0	0
Derivative cash settlements interest income (expense)	(27,869)	10,785
Interest expense	(27,869)	10,785
Net interest income	(27,869)	10,785
Provision for credit losses	0	0
Net interest income after provision for credit losses	(27,869)	10,785
Non-interest income:
Fee and other income	0	0
Derivative gains:
Derivative gains	189,887	93,587
Investment securities gains	0	0
Total non-interest income	189,887	93,587
Non-interest expense:
General and administrative expenses	0	0
Losses on early extinguishment of debt	0
Other non-interest expense	0	0
Total non-interest expense	0	0
Income before income taxes	162,018	104,372
Income tax provision	0	0
Net income (loss)	162,018	104,372
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	27,869	(10,785)
Derivative forward value gains	162,018	104,372
Intersegment eliminations
Statement of operations:
Interest income	(16,706)	(9,942)
Interest expense	16,706	9,942
Derivative cash settlements interest income (expense)	0	0
Interest expense	16,706	9,942
Net interest income	0	0
Provision for credit losses	902	247
Net interest income after provision for credit losses	902	247
Non-interest income:
Fee and other income	(3,456)	(2,656)
Derivative gains:
Derivative gains	0	0
Investment securities gains	0	0
Total non-interest income	(3,456)	(2,656)
Non-interest expense:
General and administrative expenses	2,196	2,028
Losses on early extinguishment of debt	0
Other non-interest expense	358	381
Total non-interest expense	2,554	2,409
Income before income taxes	0	0
Income tax provision	0	0
Net income (loss)	0	0
Assets:
Total loans outstanding	(1,442,215)	(1,178,809)
Deferred loan origination costs	0	0
Loans to members	(1,442,215)	(1,178,809)
Less: Allowance for credit losses	4,572	2,982
Loans to members, net	(1,437,643)	(1,175,827)
Other assets	(68,304)	(87,970)
Total assets	(1,505,947)	(1,263,797)
Intersegment eliminations | Interest rate swaps
Derivative gains:
Derivative cash settlements interest income	0	0
Derivative forward value gains	0	0
CFC | Operating segments
Statement of operations:
Interest income	378,630	304,984
Interest expense	(316,273)	(209,468)
Derivative cash settlements interest income (expense)	27,837	(10,528)
Interest expense	(288,436)	(219,996)
Net interest income	90,194	84,988
Provision for credit losses	(800)	(3,496)
Net interest income after provision for credit losses	89,394	81,492
Non-interest income:
Fee and other income	6,327	5,793
Derivative gains:
Derivative gains	0	0
Investment securities gains	2,933	(3,679)
Total non-interest income	9,260	2,114
Non-interest expense:
General and administrative expenses	(30,936)	(25,012)
Losses on early extinguishment of debt	(939)
Other non-interest expense	(177)	(322)
Total non-interest expense	(32,052)	(25,334)
Income before income taxes	66,602	58,272
Income tax provision	0	0
Net income (loss)	66,602	58,272
Assets:
Total loans outstanding	33,063,285	30,656,301
Deferred loan origination costs	13,153	12,335
Loans to members	33,076,438	30,668,636
Less: Allowance for credit losses	(54,926)	(71,056)
Loans to members, net	33,021,512	30,597,580
Other assets	1,644,426	1,456,295
Total assets	34,665,938	32,053,875
CFC | Operating segments | Interest rate swaps
Derivative gains:
Derivative cash settlements interest income	0	0
Derivative forward value gains	0	0
NCSC and RTFC | Operating segments
Statement of operations:
Interest income	19,032	11,936
Interest expense	(16,714)	(9,942)
Derivative cash settlements interest income (expense)	32	(257)
Interest expense	(16,682)	(10,199)
Net interest income	2,350	1,737
Provision for credit losses	(902)	(247)
Net interest income after provision for credit losses	1,448	1,490
Non-interest income:
Fee and other income	1,666	919
Derivative gains:
Derivative gains	0	0
Investment securities gains	0	0
Total non-interest income	1,666	919
Non-interest expense:
General and administrative expenses	(2,763)	(2,535)
Losses on early extinguishment of debt	0
Other non-interest expense	(359)	(381)
Total non-interest expense	(3,122)	(2,916)
Income before income taxes	(8)	(507)
Income tax provision	(328)	(263)
Net income (loss)	(336)	(770)
Assets:
Total loans outstanding	1,462,423	1,197,504
Deferred loan origination costs	0	0
Loans to members	1,462,423	1,197,504
Less: Allowance for credit losses	(4,572)	(2,982)
Loans to members, net	1,457,851	1,194,522
Other assets	76,321	97,694
Total assets	1,534,172	1,292,216
NCSC and RTFC | Operating segments | Interest rate swaps
Derivative gains:
Derivative cash settlements interest income	0	0
Derivative forward value gains	$ 0	$ 0