Loans - Internal Risk Rating (Details) - USD ($) $ in Thousands	Aug. 31, 2022	May 31, 2022	Aug. 31, 2021
Credit Quality
YTD Q1 2023	$ 814,875
2022	3,012,448
2021	2,371,299
2020	2,345,393
2019	1,609,624
Prior	17,998,806
Revolving Loans	2,522,551
Total	30,674,996	$ 30,051,354	$ 28,870,758
Parent Company
Credit Quality
Total	29,477,492	28,872,875
Parent Company | Distribution
Credit Quality
Total	24,244,193	23,844,242
Parent Company | Power supply
Credit Quality
Total	5,101,682	4,901,770
Parent Company | Statewide and associate
Credit Quality
Total	131,617	126,863
NCSC
Credit Quality
Total	727,869	710,878
RTFC
Credit Quality
Total	469,635	467,601
Pass
Credit Quality
YTD Q1 2023	814,875
2022	3,012,448
2021	2,366,438
2020	2,345,393
2019	1,599,581
Prior	17,840,876
Revolving Loans	2,203,759
Total	30,183,370	29,556,981
Pass | Parent Company
Credit Quality
YTD Q1 2023	791,075
2022	2,873,338
2021	2,243,934
2020	2,071,991
2019	1,585,975
Prior	17,366,886
Revolving Loans	2,056,633
Total	28,989,832	28,382,594
Pass | Parent Company | Distribution
Credit Quality
YTD Q1 2023	599,475
2022	2,472,643
2021	1,684,739
2020	1,868,407
2019	1,184,184
Prior	14,533,691
Revolving Loans	1,645,051
Total	23,988,190	23,596,004
Pass | Parent Company | Power supply
Credit Quality
YTD Q1 2023	185,300
2022	366,847
2021	557,006
2020	186,210
2019	398,612
Prior	2,814,019
Revolving Loans	375,878
Total	4,883,872	4,673,980
Pass | Parent Company | Statewide and associate
Credit Quality
YTD Q1 2023	6,300
2022	33,848
2021	2,189
2020	17,374
2019	3,179
Prior	19,176
Revolving Loans	35,704
Total	117,770	112,610
Pass | NCSC
Credit Quality
YTD Q1 2023	3,875
2022	48,958
2021	38,989
2020	229,706
2019	3,987
Prior	270,736
Revolving Loans	131,618
Total	727,869	710,878
Pass | RTFC
Credit Quality
YTD Q1 2023	19,925
2022	90,152
2021	83,515
2020	43,696
2019	9,619
Prior	203,254
Revolving Loans	15,508
Total	465,669	463,509
Special mention
Credit Quality
YTD Q1 2023	0
2022	0
2021	4,861
2020	0
2019	10,043
Prior	25,720
Revolving Loans	233,192
Total	273,816	266,583
Special mention | Parent Company
Credit Quality
YTD Q1 2023	0
2022	0
2021	4,861
2020	0
2019	10,043
Prior	21,754
Revolving Loans	233,192
Total	269,850	262,491
Special mention | Parent Company | Distribution
Credit Quality
YTD Q1 2023	0
2022	0
2021	4,861
2020	0
2019	5,078
Prior	12,872
Revolving Loans	233,192
Total	256,003	248,238
Special mention | Parent Company | Statewide and associate
Credit Quality
YTD Q1 2023	0
2022	0
2021	0
2020	0
2019	4,965
Prior	8,882
Revolving Loans	0
Total	13,847	14,253
Special mention | RTFC
Credit Quality
YTD Q1 2023	0
2022	0
2021	0
2020	0
2019	0
Prior	3,966
Revolving Loans	0
Total	3,966	4,092
Substandard
Credit Quality
YTD Q1 2023	0
2022	0
2021	0
2020	0
2019	0
Prior	0
Revolving Loans	0
Total	0	0
Doubtful
Credit Quality
YTD Q1 2023	0
2022	0
2021	0
2020	0
2019	0
Prior	132,210
Revolving Loans	85,600
Total	217,810	227,790
Doubtful | Parent Company | Power supply
Credit Quality
YTD Q1 2023	0
2022	0
2021	0
2020	0
2019	0
Prior	132,210
Revolving Loans	85,600
Total	217,810	227,790
Criticized
Credit Quality
YTD Q1 2023	0
2022	0
2021	4,861
2020	0
2019	10,043
Prior	157,930
Revolving Loans	318,792
Total	$ 491,626	$ 494,373