Allowance for Credit Losses - Recorded Investments (Details) - USD ($) $ in Thousands	Aug. 31, 2023	May 31, 2023	Aug. 31, 2022	May 31, 2022
Allowance components:
Collective allowance	$ 29,792	$ 27,335
Asset-specific allowance	25,134	25,759
Total allowance for credit losses	54,926	53,094	$ 71,056	$ 67,560
Loans outstanding:
Collectively evaluated loans	32,990,830	32,398,910
Individually evaluated loans	92,663	120,439
Total	$ 33,083,493	$ 32,519,349	30,674,996
Financing receivable, allowance for coverage ratios	0.17%	0.16%
Deferred loan origination costs	$ 13,153		12,335
Collective allowance
Loans outstanding:
Financing receivable, allowance for coverage ratios	0.09%	0.08%
Asset-specific allowance
Loans outstanding:
Financing receivable, allowance for coverage ratios	27.12%	21.39%
Parent Company
Allowance components:
Collective allowance	$ 25,505	$ 23,955
Asset-specific allowance	24,849	25,469
Total allowance for credit losses	50,354	49,424	68,074	64,825
Loans outstanding:
Collectively evaluated loans	31,531,874	30,957,840
Individually evaluated loans	89,196	116,847
Total	$ 31,621,070	$ 31,074,687
Financing receivable, allowance for coverage ratios	0.16%	0.16%
Deferred loan origination costs	$ 13,000	$ 13,000
Parent Company | Collective allowance
Loans outstanding:
Financing receivable, allowance for coverage ratios	0.08%	0.08%
Parent Company | Asset-specific allowance
Loans outstanding:
Financing receivable, allowance for coverage ratios	27.86%	21.80%
Parent Company | Distribution
Allowance components:
Collective allowance	$ 15,722	$ 14,924
Asset-specific allowance	0	0
Total allowance for credit losses	15,722	14,924	16,412	15,781
Loans outstanding:
Collectively evaluated loans	25,814,015	25,432,439
Individually evaluated loans	4,209	4,638
Total	$ 25,818,224	$ 25,437,077
Financing receivable, allowance for coverage ratios	0.06%	0.06%
Parent Company | Distribution | Collective allowance
Loans outstanding:
Financing receivable, allowance for coverage ratios	0.06%	0.06%
Parent Company | Distribution | Asset-specific allowance
Loans outstanding:
Financing receivable, allowance for coverage ratios	0.00%	0.00%
Parent Company | Power supply
Allowance components:
Collective allowance	$ 8,585	$ 7,837
Asset-specific allowance	24,849	25,469
Total allowance for credit losses	33,434	33,306	50,343	47,793
Loans outstanding:
Collectively evaluated loans	5,485,567	5,325,033
Individually evaluated loans	84,987	112,209
Total	$ 5,570,554	$ 5,437,242
Financing receivable, allowance for coverage ratios	0.60%	0.61%
Parent Company | Power supply | Collective allowance
Loans outstanding:
Financing receivable, allowance for coverage ratios	0.16%	0.15%
Parent Company | Power supply | Asset-specific allowance
Loans outstanding:
Financing receivable, allowance for coverage ratios	29.24%	22.70%
Parent Company | Statewide and associate
Allowance components:
Collective allowance	$ 1,198	$ 1,194
Asset-specific allowance	0	0
Total allowance for credit losses	1,198	1,194	1,319	1,251
Loans outstanding:
Collectively evaluated loans	232,292	200,368
Individually evaluated loans	0	0
Total	$ 232,292	$ 200,368
Financing receivable, allowance for coverage ratios	0.52%	0.60%
Parent Company | Statewide and associate | Collective allowance
Loans outstanding:
Financing receivable, allowance for coverage ratios	0.52%	0.60%
Parent Company | Statewide and associate | Asset-specific allowance
Loans outstanding:
Financing receivable, allowance for coverage ratios	0.00%	0.00%
NCSC
Allowance components:
Collective allowance	$ 2,612	$ 2,464
Asset-specific allowance	0	0
Total allowance for credit losses	2,612	2,464	1,565	1,449
Loans outstanding:
Collectively evaluated loans	923,447	956,874
Individually evaluated loans	0	0
Total	$ 923,447	$ 956,874
Financing receivable, allowance for coverage ratios	0.28%	0.26%
NCSC | Collective allowance
Loans outstanding:
Financing receivable, allowance for coverage ratios	0.28%	0.26%
NCSC | Asset-specific allowance
Loans outstanding:
Financing receivable, allowance for coverage ratios	0.00%	0.00%
RTFC
Allowance components:
Collective allowance	$ 1,675	$ 916
Asset-specific allowance	285	290
Total allowance for credit losses	1,960	1,206	$ 1,417	$ 1,286
Loans outstanding:
Collectively evaluated loans	535,509	484,196
Individually evaluated loans	3,467	3,592
Total	$ 538,976	$ 487,788
Financing receivable, allowance for coverage ratios	0.36%	0.25%
RTFC | Collective allowance
Loans outstanding:
Financing receivable, allowance for coverage ratios	0.31%	0.19%
RTFC | Asset-specific allowance
Loans outstanding:
Financing receivable, allowance for coverage ratios	8.22%	8.07%