Allowance for Credit Losses - Schedule of changes in allowance for credit losses (Details) - USD ($)	12 Months Ended
	May 31, 2024	May 31, 2023	May 31, 2022
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at the beginning of the period	$ 53,094,000	$ 67,560,000	$ 85,532,000
Provision (benefit) for credit losses	(5,516,000)	603,000	(17,972,000)
Recoveries	1,148,000		0
Charge-offs	0	(15,069,000)	0
Balance at the end of the period	48,726,000	53,094,000	67,560,000
CFC
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at the beginning of the period	49,424,000	64,825,000	79,463,000
Provision (benefit) for credit losses	(7,846,000)	(332,000)	(14,638,000)
Recoveries	1,148,000
Charge-offs		(15,069,000)
Balance at the end of the period	42,726,000	49,424,000	64,825,000
CFC | Distribution
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at the beginning of the period	14,924,000	15,781,000	13,426,000
Provision (benefit) for credit losses	1,030,000	(857,000)	2,355,000
Recoveries	0
Charge-offs		0
Balance at the end of the period	15,954,000	14,924,000	15,781,000
CFC | Power supply
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at the beginning of the period	33,306,000	47,793,000	64,646,000
Provision (benefit) for credit losses	(8,871,000)	582,000	(16,853,000)
Recoveries	1,148,000
Charge-offs		(15,069,000)
Balance at the end of the period	25,583,000	33,306,000	47,793,000
CFC | Statewide and associate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at the beginning of the period	1,194,000	1,251,000	1,391,000
Provision (benefit) for credit losses	(5,000)	(57,000)	(140,000)
Recoveries	0
Charge-offs		0
Balance at the end of the period	1,189,000	1,194,000	1,251,000
NCSC
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at the beginning of the period	3,670,000	2,735,000	6,069,000
Provision (benefit) for credit losses	2,330,000	935,000	(3,334,000)
Recoveries	0
Charge-offs		0
Balance at the end of the period	6,000,000	3,670,000	2,735,000
NCSC | Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at the beginning of the period	2,464,000	1,449,000	1,374,000
Provision (benefit) for credit losses	1,473,000	1,015,000	75,000
Recoveries	0
Charge-offs		0
Balance at the end of the period	3,937,000	2,464,000	1,449,000
NCSC | Telecom
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at the beginning of the period	1,206,000	1,286,000	4,695,000
Provision (benefit) for credit losses	857,000	(80,000)	(3,409,000)
Recoveries	0
Charge-offs		0
Balance at the end of the period	$ 2,063,000	$ 1,206,000	$ 1,286,000