Loans - Payment Status of Loans Outstanding (Details) - USD ($) $ in Thousands	Aug. 31, 2022	May 31, 2022	Aug. 31, 2021
Payment Status of Loans
Amount	$ 30,674,996	$ 30,051,354	$ 28,870,758
Nonaccrual Loans	$ 217,810	$ 227,790
Current (percent)	99.63%	99.62%
30-89 days past due (in percent)	0.00%	0.09%
> 90 days past due (in percent)	0.37%	0.29%
Total past due (in percent)	0.37%	0.38%
Total loans outstanding (in percent)	100.00%	100.00%
Nonaccrual loans (in percent)	0.71%	0.76%
Parent Company
Payment Status of Loans
Amount	$ 29,477,492	$ 28,872,875
Nonaccrual Loans	217,810	227,790
Parent Company | Distribution
Payment Status of Loans
Amount	24,244,193	23,844,242
Nonaccrual Loans	0	0
Parent Company | Power supply
Payment Status of Loans
Amount	5,101,682	4,901,770
Nonaccrual Loans	217,810	227,790
Parent Company | Statewide and associate
Payment Status of Loans
Amount	131,617	126,863
Nonaccrual Loans	0	0
NCSC
Payment Status of Loans
Amount	727,869	710,878
Nonaccrual Loans	0	0
RTFC
Payment Status of Loans
Amount	469,635	467,601
Nonaccrual Loans	0	0
Current
Payment Status of Loans
Amount	30,561,007	29,937,416
Current | Parent Company
Payment Status of Loans
Amount	29,363,503	28,758,937
Current | Parent Company | Distribution
Payment Status of Loans
Amount	24,244,193	23,844,242
Current | Parent Company | Power supply
Payment Status of Loans
Amount	4,987,693	4,787,832
Current | Parent Company | Statewide and associate
Payment Status of Loans
Amount	131,617	126,863
Current | NCSC
Payment Status of Loans
Amount	727,869	710,878
Current | RTFC
Payment Status of Loans
Amount	469,635	467,601
Total Past Due
Payment Status of Loans
Amount	113,989	113,938
Total Past Due | Parent Company
Payment Status of Loans
Amount	113,989	113,938
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	113,989	113,938
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	0
Total Past Due | RTFC
Payment Status of Loans
Amount	0	0
30-89 Days Past Due
Payment Status of Loans
Amount	51	28,389
30-89 Days Past Due | Parent Company
Payment Status of Loans
Amount	51	28,389
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	51	28,389
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	0
30-89 Days Past Due | RTFC
Payment Status of Loans
Amount	0	0
> 90 Days Past Due
Payment Status of Loans
Amount	113,938	85,549
> 90 Days Past Due | Parent Company
Payment Status of Loans
Amount	113,938	85,549
> 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	113,938	85,549
> 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