Allowance for Credit Losses - Schedule of changes in allowance for credit losses (Details) - USD ($)	12 Months Ended
	May 31, 2026	May 31, 2025	May 31, 2024
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at the beginning of the period	$ 40,615,000	$ 48,726,000	$ 53,094,000
Provision (benefit) for credit losses	(10,362,000)	(8,111,000)	(5,516,000)
Charge-offs	(306,000)	0
Recoveries			1,148,000
Balance at the end of the period	29,947,000	40,615,000	48,726,000
CFC
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at the beginning of the period	35,029,000	42,726,000	49,424,000
Provision (benefit) for credit losses	(10,467,000)	(7,697,000)	(7,846,000)
Charge-offs	(306,000)
Recoveries			1,148,000
Balance at the end of the period	24,256,000	35,029,000	42,726,000
CFC | Distribution
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at the beginning of the period	18,473,000	15,954,000	14,924,000
Provision (benefit) for credit losses	(1,848,000)	2,519,000	1,030,000
Charge-offs	0
Recoveries			0
Balance at the end of the period	16,625,000	18,473,000	15,954,000
CFC | Power supply
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at the beginning of the period	15,456,000	25,583,000	33,306,000
Provision (benefit) for credit losses	(8,489,000)	(10,127,000)	(8,871,000)
Charge-offs	(306,000)
Recoveries			1,148,000
Balance at the end of the period	6,661,000	15,456,000	25,583,000
CFC | Statewide and associate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at the beginning of the period	1,100,000	1,189,000	1,194,000
Provision (benefit) for credit losses	(130,000)	(89,000)	(5,000)
Charge-offs	0
Recoveries			0
Balance at the end of the period	970,000	1,100,000	1,189,000
NCSC
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at the beginning of the period	5,586,000	6,000,000	3,670,000
Provision (benefit) for credit losses	105,000	(414,000)	2,330,000
Charge-offs	0
Recoveries			0
Balance at the end of the period	5,691,000	5,586,000	6,000,000
NCSC | Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at the beginning of the period	3,818,000	3,937,000	2,464,000
Provision (benefit) for credit losses	290,000	(119,000)	1,473,000
Recoveries			0
Balance at the end of the period	4,108,000	3,818,000	3,937,000
NCSC | Telecom
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at the beginning of the period	1,768,000	2,063,000	1,206,000
Provision (benefit) for credit losses	(185,000)	(295,000)	857,000
Charge-offs	0
Recoveries			0
Balance at the end of the period	$ 1,583,000	$ 1,768,000	$ 2,063,000