Allowance for Credit Losses - Recorded Investments (Details) - USD ($) $ in Thousands	Nov. 30, 2021	Aug. 31, 2021	May 31, 2021	Nov. 30, 2020	Aug. 31, 2020	May 31, 2020
Allowance components:
Collective allowance	$ 46,074		$ 42,442
Asset-specific allowance	40,061		43,090
Total allowance for credit losses	86,135	$ 89,535	85,532	$ 58,989	$ 57,351	$ 53,125
Loans outstanding:
Collectively evaluated loans	28,705,936		28,167,639
Individually evaluated loans	228,878		247,468
Total	$ 28,934,814		$ 28,415,107	27,051,163
Financing receivable, allowance for credit loss to outstanding	0.30%		0.30%
Deferred loan origination costs	$ 12,056			11,806
Collective allowance
Loans outstanding:
Financing receivable, allowance for credit loss to outstanding	0.16%		0.15%
Asset-specific allowance
Loans outstanding:
Financing receivable, allowance for credit loss to outstanding	17.50%		17.41%
Parent Company
Allowance components:
Collective allowance	$ 43,265		$ 39,921
Asset-specific allowance	39,658		39,542
Total allowance for credit losses	82,923	83,260	79,463	54,409	52,730	47,438
Loans outstanding:
Collectively evaluated loans	27,557,338		27,054,165
Individually evaluated loans	224,536		233,691
Total	$ 27,781,874		$ 27,287,856
Financing receivable, allowance for credit loss to outstanding	0.30%		0.29%
Deferred loan origination costs	$ 12,000		$ 12,000
Parent Company | Collective allowance
Loans outstanding:
Financing receivable, allowance for credit loss to outstanding	0.16%		0.15%
Parent Company | Asset-specific allowance
Loans outstanding:
Financing receivable, allowance for credit loss to outstanding	17.66%		16.92%
Parent Company | Distribution
Allowance components:
Collective allowance	$ 16,032		$ 13,426
Asset-specific allowance	0		0
Total allowance for credit losses	16,032	15,369	13,426	13,215	12,037	8,002
Loans outstanding:
Collectively evaluated loans	22,552,985		22,022,044
Individually evaluated loans	5,092		5,379
Total	$ 22,558,077		$ 22,027,423
Financing receivable, allowance for credit loss to outstanding	0.07%		0.06%
Parent Company | Distribution | Collective allowance
Loans outstanding:
Financing receivable, allowance for credit loss to outstanding	0.07%		0.06%
Parent Company | Distribution | Asset-specific allowance
Loans outstanding:
Financing receivable, allowance for credit loss to outstanding	0.00%		0.00%
Parent Company | Power supply
Allowance components:
Collective allowance	$ 25,809		$ 25,104
Asset-specific allowance	39,658		39,542
Total allowance for credit losses	65,467	66,469	64,646	39,781	39,282	38,027
Loans outstanding:
Collectively evaluated loans	4,902,962		4,926,000
Individually evaluated loans	219,444		228,312
Total	$ 5,122,406		$ 5,154,312
Financing receivable, allowance for credit loss to outstanding	1.28%		1.25%
Parent Company | Power supply | Collective allowance
Loans outstanding:
Financing receivable, allowance for credit loss to outstanding	0.53%		0.51%
Parent Company | Power supply | Asset-specific allowance
Loans outstanding:
Financing receivable, allowance for credit loss to outstanding	18.07%		17.32%
Parent Company | Statewide and associate
Allowance components:
Collective allowance	$ 1,424		$ 1,391
Asset-specific allowance	0		0
Total allowance for credit losses	1,424	1,422	1,391	1,413	1,411	1,409
Loans outstanding:
Collectively evaluated loans	101,391		106,121
Individually evaluated loans	0		0
Total	$ 101,391		$ 106,121
Financing receivable, allowance for credit loss to outstanding	1.40%		1.31%
Parent Company | Statewide and associate | Collective allowance
Loans outstanding:
Financing receivable, allowance for credit loss to outstanding	1.40%		1.31%
Parent Company | Statewide and associate | Asset-specific allowance
Loans outstanding:
Financing receivable, allowance for credit loss to outstanding	0.00%		0.00%
NCSC
Allowance components:
Collective allowance	$ 1,594		$ 1,374
Asset-specific allowance	0		0
Total allowance for credit losses	1,594	1,455	1,374	1,341	829	806
Loans outstanding:
Collectively evaluated loans	721,299		706,868
Individually evaluated loans	0		0
Total	$ 721,299		$ 706,868
Financing receivable, allowance for credit loss to outstanding	0.22%		0.19%
NCSC | Collective allowance
Loans outstanding:
Financing receivable, allowance for credit loss to outstanding	0.22%		0.19%
NCSC | Asset-specific allowance
Loans outstanding:
Financing receivable, allowance for credit loss to outstanding	0.00%		0.00%
RTFC
Allowance components:
Collective allowance	$ 1,215		$ 1,147
Asset-specific allowance	403		3,548
Total allowance for credit losses	1,618	$ 4,820	4,695	$ 3,239	$ 3,792	$ 4,881
Loans outstanding:
Collectively evaluated loans	427,299		406,606
Individually evaluated loans	4,342		13,777
Total	$ 431,641		$ 420,383
Financing receivable, allowance for credit loss to outstanding	0.37%		1.12%
RTFC | Collective allowance
Loans outstanding:
Financing receivable, allowance for credit loss to outstanding	0.28%		0.28%
RTFC | Asset-specific allowance
Loans outstanding:
Financing receivable, allowance for credit loss to outstanding	9.28%		25.75%