Loans - Internal Risk Rating (Details) - USD ($) $ in Thousands	Feb. 28, 2022	May 31, 2021	Feb. 28, 2021
Credit Quality
YTD Q3 2022	$ 2,115,798
2021	2,416,619
2020	2,387,162
2019	1,646,385
2018	1,781,148
Prior	16,825,257
Revolving Loans	2,335,994
Total	29,508,363	$ 28,415,107	$ 28,315,671
Parent Company
Credit Quality
Total	28,320,476	27,287,856
Parent Company | Distribution
Credit Quality
Total	23,225,751	22,027,423
Parent Company | Power supply
Credit Quality
Total	4,992,073	5,154,312
Parent Company | Statewide and associate
Credit Quality
Total	102,652	106,121
NCSC
Credit Quality
Total	730,147	706,868
RTFC
Credit Quality
Total	457,740	420,383
Pass
Credit Quality
YTD Q3 2022	2,115,798
2021	2,411,702
2020	2,387,162
2019	1,636,259
2018	1,776,319
Prior	16,652,951
Revolving Loans	2,023,799
Total	29,003,990	27,529,242
Pass | Parent Company
Credit Quality
YTD Q3 2022	2,047,496
2021	2,280,256
2020	2,106,317
2019	1,621,545
2018	1,709,675
Prior	16,222,646
Revolving Loans	1,832,385
Total	27,820,320	26,415,768
Pass | Parent Company | Distribution
Credit Quality
YTD Q3 2022	1,714,893
2021	1,715,341
2020	1,895,645
2019	1,201,281
2018	1,462,991
Prior	13,521,337
Revolving Loans	1,464,191
Total	22,975,679	21,808,099
Pass | Parent Company | Power supply
Credit Quality
YTD Q3 2022	329,172
2021	562,604
2020	191,205
2019	416,879
2018	246,684
Prior	2,680,286
Revolving Loans	329,600
Total	4,756,430	4,517,408
Pass | Parent Company | Statewide and associate
Credit Quality
YTD Q3 2022	3,431
2021	2,311
2020	19,467
2019	3,385
2018	0
Prior	21,023
Revolving Loans	38,594
Total	88,211	90,261
Pass | NCSC
Credit Quality
YTD Q3 2022	14,251
2021	40,152
2020	234,522
2019	4,147
2018	43,140
Prior	243,347
Revolving Loans	150,588
Total	730,147	706,868
Pass | RTFC
Credit Quality
YTD Q3 2022	54,051
2021	91,294
2020	46,323
2019	10,567
2018	23,504
Prior	186,958
Revolving Loans	40,826
Total	453,523	406,606
Special mention
Credit Quality
YTD Q3 2022	0
2021	4,917
2020	0
2019	10,126
2018	4,829
Prior	50,601
Revolving Loans	226,646
Total	297,119	269,387
Special mention | Parent Company
Credit Quality
YTD Q3 2022	0
2021	4,917
2020	0
2019	10,126
2018	4,829
Prior	46,384
Revolving Loans	226,646
Total	292,902	264,795
Special mention | Parent Company | Distribution
Credit Quality
YTD Q3 2022	0
2021	4,917
2020	0
2019	5,126
2018	937
Prior	12,446
Revolving Loans	226,646
Total	250,072	219,324
Special mention | Parent Company | Power supply
Credit Quality
YTD Q3 2022	0
2021	0
2020	0
2019	0
2018	0
Prior	28,389
Revolving Loans	0
Total	28,389	29,611
Special mention | Parent Company | Statewide and associate
Credit Quality
YTD Q3 2022	0
2021	0
2020	0
2019	5,000
2018	3,892
Prior	5,549
Revolving Loans	0
Total	14,441	15,860
Special mention | RTFC
Credit Quality
YTD Q3 2022	0
2021	0
2020	0
2019	0
2018	0
Prior	4,217
Revolving Loans	0
Total	4,217	4,592
Substandard
Credit Quality
YTD Q3 2022	0
2021	0
2020	0
2019	0
2018	0
Prior	0
Revolving Loans	0
Total	0	378,981
Substandard | Parent Company | Power supply
Credit Quality
YTD Q3 2022	0
2021	0
2020	0
2019	0
2018	0
Prior	0
Revolving Loans	0
Total	0	378,981
Doubtful
Credit Quality
YTD Q3 2022	0
2021	0
2020	0
2019	0
2018	0
Prior	121,705
Revolving Loans	85,549
Total	207,254	237,497
Doubtful | Parent Company
Credit Quality
YTD Q3 2022	0
2021	0
2020	0
2019	0
2018	0
Prior	121,705
Revolving Loans	85,549
Total	207,254	228,312
Doubtful | Parent Company | Power supply
Credit Quality
YTD Q3 2022	0
2021	0
2020	0
2019	0
2018	0
Prior	121,705
Revolving Loans	85,549
Total	207,254	228,312
Doubtful | RTFC
Credit Quality
YTD Q3 2022	0
2021	0
2020	0
2019	0
2018	0
Prior	0
Revolving Loans	0
Total	0	9,185
Criticized
Credit Quality
YTD Q3 2022	0
2021	4,917
2020	0
2019	10,126
2018	4,829
Prior	172,306
Revolving Loans	312,195
Total	$ 504,373	$ 885,865