Allowance for Credit Losses - Rollforward (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Nov. 30, 2021	Nov. 30, 2020	Nov. 30, 2021	Nov. 30, 2020
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan loss, beginning balance	$ 89,535	$ 57,351	$ 85,532	$ 53,125
Provision (benefit) for credit losses	(3,400)	1,638	603	1,964
Allowance for loan loss, ending balance	86,135	58,989	86,135	58,989
Parent Company
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan loss, beginning balance	83,260	52,730	79,463	47,438
Provision (benefit) for credit losses	(337)	1,679	3,460	1,326
Allowance for loan loss, ending balance	82,923	54,409	82,923	54,409
Parent Company | Distribution
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan loss, beginning balance	15,369	12,037	13,426	8,002
Provision (benefit) for credit losses	663	1,178	2,606	1,627
Allowance for loan loss, ending balance	16,032	13,215	16,032	13,215
Parent Company | Power supply
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan loss, beginning balance	66,469	39,282	64,646	38,027
Provision (benefit) for credit losses	(1,002)	499	821	(280)
Allowance for loan loss, ending balance	65,467	39,781	65,467	39,781
Parent Company | Statewide and associate
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan loss, beginning balance	1,422	1,411	1,391	1,409
Provision (benefit) for credit losses	2	2	33	(21)
Allowance for loan loss, ending balance	1,424	1,413	1,424	1,413
NCSC
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan loss, beginning balance	1,455	829	1,374	806
Provision (benefit) for credit losses	139	512	220	550
Allowance for loan loss, ending balance	1,594	1,341	1,594	1,341
RTFC
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan loss, beginning balance	4,820	3,792	4,695	4,881
Provision (benefit) for credit losses	(3,202)	(553)	(3,077)	(88)
Allowance for loan loss, ending balance	$ 1,618	$ 3,239	$ 1,618	3,239
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)