Loans - Payment Status of Loans Outstanding (Details) - USD ($) $ in Thousands	Aug. 31, 2023	May 31, 2023	Aug. 31, 2022
Payment Status of Loans
Amount	$ 33,083,493	$ 32,519,349	$ 30,674,996
Nonaccrual Loans	$ 84,987	$ 112,209
Current, Percentage of total loans	100.00%	99.87%
30-89 days past due, Percentage of total loans	0.00%	0.11%
> 90 days past due, Percentage of total loans	0.00%	0.02%
Total past due, Percentage of total loans	0.00%	0.13%
Total loans outstanding, Percentage of total loans	100.00%	100.00%
Nonaccrual loans, Percentage of total loans	0.26%	0.35%
Parent Company
Payment Status of Loans
Amount	$ 31,621,070	$ 31,074,687
Nonaccrual Loans	84,987	112,209
Parent Company | Distribution
Payment Status of Loans
Amount	25,818,224	25,437,077
Nonaccrual Loans	0	0
Parent Company | Power supply
Payment Status of Loans
Amount	5,570,554	5,437,242
Nonaccrual Loans	84,987	112,209
Parent Company | Statewide and associate
Payment Status of Loans
Amount	232,292	200,368
Nonaccrual Loans	0	0
NCSC
Payment Status of Loans
Amount	923,447	956,874
Nonaccrual Loans	0	0
RTFC
Payment Status of Loans
Amount	538,976	487,788
Nonaccrual Loans	0	0
Current
Payment Status of Loans
Amount	33,083,493	32,478,287
Current | Parent Company
Payment Status of Loans
Amount	31,621,070	31,070,340
Current | Parent Company | Distribution
Payment Status of Loans
Amount	25,818,224	25,437,077
Current | Parent Company | Power supply
Payment Status of Loans
Amount	5,570,554	5,432,895
Current | Parent Company | Statewide and associate
Payment Status of Loans
Amount	232,292	200,368
Current | NCSC
Payment Status of Loans
Amount	923,447	920,159
Current | RTFC
Payment Status of Loans
Amount	538,976	487,788
Total Past Due
Payment Status of Loans
Amount	0	41,062
Total Past Due | Parent Company
Payment Status of Loans
Amount	0	4,347
Total Past Due | Parent Company | Distribution
Payment Status of Loans
Amount	0	0
Total Past Due | Parent Company | Power supply
Payment Status of Loans
Amount	0	4,347
Total Past Due | Parent Company | Statewide and associate
Payment Status of Loans
Amount	0	0
Total Past Due | NCSC
Payment Status of Loans
Amount	0	36,715
Total Past Due | RTFC
Payment Status of Loans
Amount	0	0
30-89 Days Past Due
Payment Status of Loans
Amount	0	36,715
30-89 Days Past Due | Parent Company
Payment Status of Loans
Amount	0	0
30-89 Days Past Due | Parent Company | Distribution
Payment Status of Loans
Amount	0	0
30-89 Days Past Due | Parent Company | Power supply
Payment Status of Loans
Amount	0	0
30-89 Days Past Due | Parent Company | Statewide and associate
Payment Status of Loans
Amount	0	0
30-89 Days Past Due | NCSC
Payment Status of Loans
Amount	0	36,715
30-89 Days Past Due | RTFC
Payment Status of Loans
Amount	0	0
> 90 Days Past Due
Payment Status of Loans
Amount	0	4,347
> 90 Days Past Due | Parent Company
Payment Status of Loans
Amount	0	4,347
> 90 Days Past Due | Parent Company | Distribution
Payment Status of Loans
Amount	0	0
> 90 Days Past Due | Parent Company | Power supply
Payment Status of Loans
Amount	0	4,347
> 90 Days Past Due | Parent Company | Statewide and associate
Payment Status of Loans
Amount	0	0
> 90 Days Past Due | NCSC
Payment Status of Loans
Amount	0	0
> 90 Days Past Due | RTFC
Payment Status of Loans
Amount	$ 0	$ 0