Loans - Internal Risk Rating (Details) - USD ($) $ in Thousands	Feb. 28, 2023	May 31, 2022	Feb. 28, 2022
Credit Quality
YTD Q3 2023	$ 2,458,106
2022	2,954,725
2021	2,301,877
2020	2,284,901
2019	1,573,824
Prior	17,381,470
Revolving Loans	3,414,328
Total	32,369,231	$ 30,051,354	$ 29,508,363
Parent Company
Credit Quality
Total	30,899,071	28,872,875
Parent Company | Distribution
Credit Quality
Total	25,424,628	23,844,242
Parent Company | Power supply
Credit Quality
Total	5,318,565	4,901,770
Parent Company | Statewide and associate
Credit Quality
Total	155,878	126,863
NCSC
Credit Quality
Total	988,371	710,878
RTFC
Credit Quality
Total	481,789	467,601
Pass
Credit Quality
YTD Q3 2023	2,453,866
2022	2,954,725
2021	2,297,073
2020	2,284,901
2019	1,563,952
Prior	17,248,693
Revolving Loans	3,228,778
Total	32,031,988	29,556,981
Pass | Parent Company
Credit Quality
YTD Q3 2023	2,167,441
2022	2,819,698
2021	2,213,425
2020	2,033,908
2019	1,551,742
Prior	16,796,286
Revolving Loans	2,983,045
Total	30,565,545	28,382,594
Pass | Parent Company | Distribution
Credit Quality
YTD Q3 2023	1,800,130
2022	2,431,756
2021	1,660,098
2020	1,836,617
2019	1,162,421
Prior	14,049,245
Revolving Loans	2,295,219
Total	25,235,486	23,596,004
Pass | Parent Company | Power supply
Credit Quality
YTD Q3 2023	343,465
2022	354,189
2021	551,263
2020	181,672
2019	386,352
Prior	2,729,956
Revolving Loans	640,626
Total	5,187,523	4,673,980
Pass | Parent Company | Statewide and associate
Credit Quality
YTD Q3 2023	23,846
2022	33,753
2021	2,064
2020	15,619
2019	2,969
Prior	17,085
Revolving Loans	47,200
Total	142,536	112,610
Pass | NCSC
Credit Quality
YTD Q3 2023	238,254
2022	48,131
2021	5,866
2020	210,308
2019	3,824
Prior	256,838
Revolving Loans	225,150
Total	988,371	710,878
Pass | RTFC
Credit Quality
YTD Q3 2023	48,171
2022	86,896
2021	77,782
2020	40,685
2019	8,386
Prior	195,569
Revolving Loans	20,583
Total	478,072	463,509
Special mention
Credit Quality
YTD Q3 2023	4,240
2022	0
2021	4,804
2020	0
2019	9,872
Prior	24,610
Revolving Loans	162,675
Total	206,201	266,583
Special mention | Parent Company
Credit Quality
YTD Q3 2023	4,240
2022	0
2021	4,804
2020	0
2019	9,872
Prior	20,893
Revolving Loans	162,675
Total	202,484	262,491
Special mention | Parent Company | Distribution
Credit Quality
YTD Q3 2023	4,240
2022	0
2021	4,804
2020	0
2019	5,028
Prior	12,395
Revolving Loans	162,675
Total	189,142	248,238
Special mention | Parent Company | Statewide and associate
Credit Quality
YTD Q3 2023	0
2022	0
2021	0
2020	0
2019	4,844
Prior	8,498
Revolving Loans	0
Total	13,342	14,253
Special mention | RTFC
Credit Quality
YTD Q3 2023	0
2022	0
2021	0
2020	0
2019	0
Prior	3,717
Revolving Loans	0
Total	3,717	4,092
Substandard
Credit Quality
YTD Q3 2023	0
2022	0
2021	0
2020	0
2019	0
Prior	0
Revolving Loans	0
Total	0	0
Doubtful
Credit Quality
YTD Q3 2023	0
2022	0
2021	0
2020	0
2019	0
Prior	108,167
Revolving Loans	22,875
Total	131,042	227,790
Doubtful | Parent Company | Power supply
Credit Quality
YTD Q3 2023	0
2022	0
2021	0
2020	0
2019	0
Prior	108,167
Revolving Loans	22,875
Total	131,042	227,790
Criticized
Credit Quality
YTD Q3 2023	4,240
2022	0
2021	4,804
2020	0
2019	9,872
Prior	132,777
Revolving Loans	185,550
Total	$ 337,243	$ 494,373