Allowance for Credit Losses - Rollforward (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Feb. 28, 2022	Feb. 28, 2021	Feb. 28, 2022	Feb. 28, 2021
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan loss, beginning balance	$ 86,135	$ 58,989	$ 85,532	$ 53,125
(Benefit) provision for credit losses	(12,749)	33,023	(12,146)	34,987
Allowance for loan loss, ending balance	73,386	92,012	73,386	92,012
Parent Company
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan loss, beginning balance	82,923	54,409	79,463	47,438
(Benefit) provision for credit losses	(12,747)	29,435	(9,287)	30,761
Allowance for loan loss, ending balance	70,176	83,844	70,176	83,844
Parent Company | Distribution
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan loss, beginning balance	16,032	13,215	13,426	8,002
(Benefit) provision for credit losses	353	2,022	2,959	3,649
Allowance for loan loss, ending balance	16,385	15,237	16,385	15,237
Parent Company | Power supply
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan loss, beginning balance	65,467	39,781	64,646	38,027
(Benefit) provision for credit losses	(12,989)	27,381	(12,168)	27,101
Allowance for loan loss, ending balance	52,478	67,162	52,478	67,162
Parent Company | Statewide and associate
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan loss, beginning balance	1,424	1,413	1,391	1,409
(Benefit) provision for credit losses	(111)	32	(78)	11
Allowance for loan loss, ending balance	1,313	1,445	1,313	1,445
NCSC
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan loss, beginning balance	1,594	1,341	1,374	806
(Benefit) provision for credit losses	135	316	355	866
Allowance for loan loss, ending balance	1,729	1,657	1,729	1,657
RTFC
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan loss, beginning balance	1,618	3,239	4,695	4,881
(Benefit) provision for credit losses	(137)	3,272	(3,214)	(3,360)
Allowance for loan loss, ending balance	$ 1,481	$ 6,511	$ 1,481	6,511
Cumulative effect, period of adoption, adjusted balance
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan loss, beginning balance				57,025
Cumulative effect, period of adoption, adjusted balance | Parent Company
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan loss, beginning balance				53,083
Cumulative effect, period of adoption, adjusted balance | Parent Company | Distribution
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan loss, beginning balance				11,588
Cumulative effect, period of adoption, adjusted balance | Parent Company | Power supply
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan loss, beginning balance				40,061
Cumulative effect, period of adoption, adjusted balance | Parent Company | Statewide and associate
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan loss, beginning balance				1,434
Cumulative effect, period of adoption, adjusted balance | NCSC
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan loss, beginning balance				791
Cumulative effect, period of adoption, adjusted balance | RTFC
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan loss, beginning balance				3,151
Cumulative-effect adjustment from adoption of CECL accounting standard
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan loss, beginning balance				3,900
Cumulative-effect adjustment from adoption of CECL accounting standard | Parent Company
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan loss, beginning balance				5,645
Cumulative-effect adjustment from adoption of CECL accounting standard | Parent Company | Distribution
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan loss, beginning balance				3,586
Cumulative-effect adjustment from adoption of CECL accounting standard | Parent Company | Power supply
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan loss, beginning balance				2,034
Cumulative-effect adjustment from adoption of CECL accounting standard | Parent Company | Statewide and associate
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan loss, beginning balance				25
Cumulative-effect adjustment from adoption of CECL accounting standard | NCSC
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan loss, beginning balance				(15)
Cumulative-effect adjustment from adoption of CECL accounting standard | RTFC
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan loss, beginning balance				$ (1,730)