Loans - Payment Status of Loans Outstanding (Details) - USD ($) $ in Thousands	Feb. 28, 2022	May 31, 2021	Feb. 28, 2021
Payment Status of Loans
Amount	$ 29,508,363	$ 28,415,107	$ 28,315,671
Nonaccrual Loans	$ 207,254	$ 237,497
Current (percent)	99.71%	99.70%
30-89 days past due (in percent)	0.00%	0.01%
> 90 days past due (in percent)	0.29%	0.29%
Total past due (in percent)	0.29%	0.30%
Total loans outstanding (in percent)	100.00%	100.00%
Nonaccrual loans (in percent)	0.70%	0.84%
Parent Company
Payment Status of Loans
Amount	$ 28,320,476	$ 27,287,856
Nonaccrual Loans	207,254	228,312
Parent Company | Distribution
Payment Status of Loans
Amount	23,225,751	22,027,423
Nonaccrual Loans	0	0
Parent Company | Power supply
Payment Status of Loans
Amount	4,992,073	5,154,312
Nonaccrual Loans	207,254	228,312
Parent Company | Statewide and associate
Payment Status of Loans
Amount	102,652	106,121
Nonaccrual Loans	0	0
NCSC
Payment Status of Loans
Amount	730,147	706,868
Nonaccrual Loans	0	0
RTFC
Payment Status of Loans
Amount	457,740	420,383
Nonaccrual Loans	0	9,185
Current
Payment Status of Loans
Amount	29,422,814	28,330,111
Current | Parent Company
Payment Status of Loans
Amount	28,234,927	27,202,860
Current | Parent Company | Distribution
Payment Status of Loans
Amount	23,225,751	22,027,423
Current | Parent Company | Power supply
Payment Status of Loans
Amount	4,906,524	5,069,316
Current | Parent Company | Statewide and associate
Payment Status of Loans
Amount	102,652	106,121
Current | NCSC
Payment Status of Loans
Amount	730,147	706,868
Current | RTFC
Payment Status of Loans
Amount	457,740	420,383
Total Past Due
Payment Status of Loans
Amount	85,549	84,996
Total Past Due | Parent Company
Payment Status of Loans
Amount	85,549	84,996
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	85,549	84,996
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	0	3,400
30-89 Days Past Due | Parent Company
Payment Status of Loans
Amount	0	3,400
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	3,400
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	85,549	81,596
> 90 Days Past Due | Parent Company
Payment Status of Loans
Amount	85,549	81,596
> 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	85,549	81,596
> 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