Loans - Internal Risk Rating (Details) - USD ($) $ in Thousands	Nov. 30, 2021	May 31, 2021	Nov. 30, 2020
Credit Quality
YTD Q2 2022	$ 1,274,335
2021	2,442,491
2020	2,407,838
2019	1,660,726
2018	1,795,954
Prior	17,121,974
Revolving Loans	2,231,496
Total	28,934,814	$ 28,415,107	$ 27,051,163
Parent Company
Credit Quality
Total	27,781,874	27,287,856
Parent Company | Distribution
Credit Quality
Total	22,558,077	22,027,423
Parent Company | Power supply
Credit Quality
Total	5,122,406	5,154,312
Parent Company | Statewide and associate
Credit Quality
Total	101,391	106,121
NCSC
Credit Quality
Total	721,299	706,868
RTFC
Credit Quality
Total	431,641	420,383
Pass
Credit Quality
YTD Q2 2022	1,274,335
2021	2,414,346
2020	2,407,838
2019	1,568,707
2018	1,791,067
Prior	16,874,382
Revolving Loans	1,714,002
Total	28,044,677	27,529,242
Pass | Parent Company
Credit Quality
YTD Q2 2022	1,249,098
2021	2,280,184
2020	2,123,354
2019	1,553,385
2018	1,722,922
Prior	16,434,729
Revolving Loans	1,532,407
Total	26,896,079	26,415,768
Pass | Parent Company | Distribution
Credit Quality
YTD Q2 2022	1,003,440
2021	1,736,046
2020	1,909,008
2019	1,211,086
2018	1,474,402
Prior	13,747,440
Revolving Loans	1,225,521
Total	22,306,943	21,808,099
Pass | Parent Company | Power supply
Credit Quality
YTD Q2 2022	244,193
2021	541,766
2020	194,019
2019	338,813
2018	248,520
Prior	2,665,332
Revolving Loans	270,648
Total	4,503,291	4,517,408
Pass | Parent Company | Statewide and associate
Credit Quality
YTD Q2 2022	1,465
2021	2,372
2020	20,327
2019	3,486
2018	0
Prior	21,957
Revolving Loans	36,238
Total	85,845	90,261
Pass | NCSC
Credit Quality
YTD Q2 2022	0
2021	40,349
2020	236,919
2019	4,225
2018	43,346
Prior	249,631
Revolving Loans	146,829
Total	721,299	706,868
Pass | RTFC
Credit Quality
YTD Q2 2022	25,237
2021	93,813
2020	47,565
2019	11,097
2018	24,799
Prior	190,022
Revolving Loans	34,766
Total	427,299	406,606
Special mention
Credit Quality
YTD Q2 2022	0
2021	4,945
2020	0
2019	10,150
2018	4,887
Prior	52,635
Revolving Loans	227,405
Total	300,022	269,387
Special mention | Parent Company
Credit Quality
YTD Q2 2022	0
2021	4,945
2020	0
2019	10,150
2018	4,887
Prior	48,293
Revolving Loans	227,405
Total	295,680	264,795
Special mention | Parent Company | Distribution
Credit Quality
YTD Q2 2022	0
2021	4,945
2020	0
2019	5,150
2018	941
Prior	12,693
Revolving Loans	227,405
Total	251,134	219,324
Special mention | Parent Company | Power supply
Credit Quality
YTD Q2 2022	0
2021	0
2020	0
2019	0
2018	0
Prior	29,000
Revolving Loans	0
Total	29,000	29,611
Special mention | Parent Company | Statewide and associate
Credit Quality
YTD Q2 2022	0
2021	0
2020	0
2019	5,000
2018	3,946
Prior	6,600
Revolving Loans	0
Total	15,546	15,860
Special mention | RTFC
Credit Quality
YTD Q2 2022	0
2021	0
2020	0
2019	0
2018	0
Prior	4,342
Revolving Loans	0
Total	4,342	4,592
Substandard
Credit Quality
YTD Q2 2022	0
2021	23,200
2020	0
2019	81,869
2018	0
Prior	61,042
Revolving Loans	204,560
Total	370,671	378,981
Substandard | Parent Company | Power supply
Credit Quality
YTD Q2 2022	0
2021	23,200
2020	0
2019	81,869
2018	0
Prior	61,042
Revolving Loans	204,560
Total	370,671	378,981
Doubtful
Credit Quality
YTD Q2 2022	0
2021	0
2020	0
2019	0
2018	0
Prior	133,915
Revolving Loans	85,529
Total	219,444	237,497
Doubtful | Parent Company
Credit Quality
YTD Q2 2022	0
2021	0
2020	0
2019	0
2018	0
Prior	133,915
Revolving Loans	85,529
Total	219,444	228,312
Doubtful | Parent Company | Power supply
Credit Quality
YTD Q2 2022	0
2021	0
2020	0
2019	0
2018	0
Prior	133,915
Revolving Loans	85,529
Total	219,444	228,312
Doubtful | RTFC
Credit Quality
YTD Q2 2022	0
2021	0
2020	0
2019	0
2018	0
Prior	0
Revolving Loans	0
Total	0	9,185
Criticized
Credit Quality
YTD Q2 2022	0
2021	28,145
2020	0
2019	92,019
2018	4,887
Prior	247,592
Revolving Loans	517,494
Total	$ 890,137	$ 885,865