Business Segments - Segment Results and Total Assets (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Nov. 30, 2019	Nov. 30, 2018	Nov. 30, 2019	Nov. 30, 2018	Aug. 31, 2019	May 31, 2019	Aug. 31, 2018	May 31, 2018
Statement of operations:
Interest income		$ 287,037	$ 281,253	$ 577,052	$ 559,744
Interest expense	[1],[2]	(207,871)	(204,166)	(421,142)	(414,397)
Net interest income		79,166	77,087	155,910	145,347
Provision for loan losses		1,045	1,788	1,015	1,897
Net interest income after benefit for loan losses		80,211	78,875	156,925	147,244
Fee and other income		3,842	3,595	14,783	7,506
Non-interest income:
Derivative gains (losses)		183,450	63,343	(212,275)	70,526
Unrealized gains (losses) on equity securities		(114)	(1,619)	1,506	(2,345)
Total non-interest income		187,178	65,319	(195,986)	75,687
Non-interest expense:
General and administrative expenses		(24,769)	(23,870)	(50,098)	(47,075)
Losses on early extinguishment of debt		614	0	614	7,100
Other non-interest (expense) income		(315)	(355)	6,864	(749)
Total non-interest expense		(25,698)	(24,225)	(43,848)	(54,924)
Income (loss) before income taxes		241,691	119,969	(82,909)	168,007
Income tax benefit (expense)		(91)	(243)	430	(303)
Net income (loss)		241,600	119,726	(82,479)	167,704
Assets:
Total recorded investment in loans		26,426,879	25,282,953	26,426,879	25,282,953		$ 25,905,664
Unamortized Loan Commitment and Origination Fees and Unamortized Discounts or Premiums		11,302	11,222	11,302	11,222		11,240
Loans to members		26,438,181	25,294,175	26,438,181	25,294,175		25,916,904
Less: Allowance for loan losses		(16,520)	(16,904)	(16,520)	(16,904)	$ (17,565)	(17,535)	$ (18,692)	$ (18,801)
Loans to members, net		26,421,661	25,277,271	26,421,661	25,277,271		25,899,369
Other assets		1,144,940	1,552,487	1,144,940	1,552,487
Total assets		27,566,601	26,829,758	27,566,601	26,829,758		$ 27,124,372
Unrealized Gain (Loss) on Derivatives				187,082	(95,160)
CFC
Statement of operations:
Interest income		285,036	279,033	573,000	555,276
Interest expense		(207,759)	(203,985)	(420,894)	(414,035)
Net interest income		77,277	75,048	152,106	141,241
Provision for loan losses		1,045	1,788	1,015	1,897
Net interest income after benefit for loan losses		78,322	76,836	153,121	143,138
Fee and other income		5,181	4,897	17,463	10,096
Non-interest income:
Derivative gains (losses)		182,513	63,045	(211,050)	70,154
Unrealized gains (losses) on equity securities		(114)	1,619	1,506	(2,345)
Total non-interest income		187,580	66,323	(192,081)	77,905
Non-interest expense:
General and administrative expenses		(23,994)	(23,544)	(48,733)	(45,969)
Losses on early extinguishment of debt		0		0	7,100
Other non-interest (expense) income		(316)	(355)	6,863	(749)
Total non-interest expense		(24,310)	(23,899)	(41,870)	(53,818)
Income (loss) before income taxes		241,592	119,260	(80,830)	167,225
Net income (loss)		241,592	119,260	(80,830)	167,225
Assets:
Total recorded investment in loans		26,406,139	25,250,000	26,406,139	25,250,000
Unamortized Loan Commitment and Origination Fees and Unamortized Discounts or Premiums		11,302	11,222	11,302	11,222
Loans to members		26,417,441	25,261,222	26,417,441	25,261,222
Less: Allowance for loan losses		(16,520)	(16,904)	(16,520)	(16,904)
Loans to members, net		26,400,921	25,244,318	26,400,921	25,244,318
Other assets		1,132,171	1,542,099	1,132,171	1,542,099
Total assets		27,533,092	26,786,417	27,533,092	26,786,417
Other
Statement of operations:
Interest income		11,719	12,945	24,066	25,929
Interest expense		(9,830)	(10,906)	(20,262)	(21,823)
Net interest income		1,889	2,039	3,804	4,106
Provision for loan losses				0
Net interest income after benefit for loan losses		1,889	2,039	3,804	4,106
Fee and other income		587	490	8,408	1,054
Non-interest income:
Derivative gains (losses)		937	298	(1,225)	372
Total non-interest income		1,524	788	7,183	1,426
Non-interest expense:
General and administrative expenses		(2,420)	(1,919)	(4,655)	(4,293)
Losses on early extinguishment of debt		(614)		(614)
Other non-interest (expense) income		(280)	(199)	(7,797)	(457)
Total non-interest expense		(3,314)	(2,118)	(13,066)	(4,750)
Income (loss) before income taxes		99	709	(2,079)	782
Income tax benefit (expense)		(91)	(243)	430	(303)
Net income (loss)		8	466	(1,649)	479
Assets:
Total recorded investment in loans		1,058,603	1,122,718	1,058,603	1,122,718
Loans to members		1,058,603	1,122,718	1,058,603	1,122,718
Loans to members, net		1,058,603	1,122,718	1,058,603	1,122,718
Other assets		106,301	112,923	106,301	112,923
Total assets		1,164,904	1,235,641	1,164,904	1,235,641
Elimination
Statement of operations:
Interest income		(9,718)	(10,725)	(20,014)	(21,461)
Interest expense		9,718	10,725	20,014	21,461
Net interest income		0	0	0	0
Provision for loan losses				0
Net interest income after benefit for loan losses		0	0	0	0
Fee and other income		(1,926)	(1,792)	(11,088)	(3,644)
Non-interest income:
Derivative gains (losses)		0	0	0	0
Total non-interest income		(1,926)	(1,792)	(11,088)	(3,644)
Non-interest expense:
General and administrative expenses		1,645	1,593	3,290	3,187
Other non-interest (expense) income		281	199	7,798	457
Total non-interest expense		1,926	1,792	11,088	3,644
Assets:
Total recorded investment in loans		(1,037,863)	(1,089,765)	(1,037,863)	(1,089,765)
Loans to members		(1,037,863)	(1,089,765)	(1,037,863)	(1,089,765)
Loans to members, net		(1,037,863)	(1,089,765)	(1,037,863)	(1,089,765)
Other assets		(93,532)	(102,535)	(93,532)	(102,535)
Total assets		(1,131,395)	(1,192,300)	(1,131,395)	(1,192,300)
Interest Rate Swap
Statement of operations:
Interest expense		(14,150)	(11,805)	(25,193)	(24,634)
Non-interest income:
Derivative gains (losses)		183,450	63,343	(212,275)	70,526
Assets:
Unrealized Gain (Loss) on Derivatives		(197,600)	(75,148)	187,082	(95,160)
Interest Rate Swap | CFC
Statement of operations:
Interest expense		(13,874)	(11,546)	24,675	24,108
Assets:
Unrealized Gain (Loss) on Derivatives		(196,387)	(74,591)	186,375	(94,262)
Interest Rate Swap | Other
Statement of operations:
Interest expense		276	259	518	526
Assets:
Unrealized Gain (Loss) on Derivatives		$ 1,213	$ 557	$ (707)	$ 898

[1] Includes amortization of debt discounts and debt issuance costs, which are generally deferred and recognized as interest expense using the effective interest method. Issuance costs related to dealer commercial paper, however, are recognized as interest expense immediately as incurred.

[2] Includes fees related to funding arrangements, such as up-front fees paid to banks participating in our committed bank revolving line of credit agreements. Depending on the nature of the fee, amounts may be deferred and recognized as interest expense ratably over the term of the arrangement or recognized immediately as incurred.