Allowance for Credit Losses - Schedule of Allowance for Credit Losses on Financing Receivables (Details) - USD ($)	3 Months Ended		9 Months Ended
	Feb. 28, 2026	Feb. 28, 2025	Feb. 28, 2026	Feb. 28, 2025
Allowance for Loan and Lease Losses [Roll Forward]
Balance at the beginning of the period	$ 43,423,000	$ 50,549,000	$ 40,615,000	$ 48,726,000
Provision (benefit) for credit losses	(7,405,000)	(6,093,000)	(4,597,000)	(4,270,000)
Charge-offs	(306,000)		(306,000)	0
Balance at the end of the period	35,712,000	44,456,000	35,712,000	44,456,000
CFC:
Allowance for Loan and Lease Losses [Roll Forward]
Balance at the beginning of the period	36,808,000	44,032,000	35,029,000	42,726,000
Provision (benefit) for credit losses	(8,042,000)	(6,369,000)	(6,263,000)	(5,063,000)
Charge-offs	(306,000)		(306,000)
Balance at the end of the period	28,460,000	37,663,000	28,460,000	37,663,000
CFC: | Distribution
Allowance for Loan and Lease Losses [Roll Forward]
Balance at the beginning of the period	20,102,000	18,049,000	18,473,000	15,954,000
Provision (benefit) for credit losses	274,000	1,452,000	1,903,000	3,547,000
Charge-offs	0		0
Balance at the end of the period	20,376,000	19,501,000	20,376,000	19,501,000
CFC: | Power supply
Allowance for Loan and Lease Losses [Roll Forward]
Balance at the beginning of the period	15,593,000	24,785,000	15,456,000	25,583,000
Provision (benefit) for credit losses	(8,272,000)	(7,790,000)	(8,135,000)	(8,588,000)
Charge-offs	(306,000)		(306,000)
Balance at the end of the period	7,015,000	16,995,000	7,015,000	16,995,000
CFC: | Statewide and associate
Allowance for Loan and Lease Losses [Roll Forward]
Balance at the beginning of the period	1,113,000	1,198,000	1,100,000	1,189,000
Provision (benefit) for credit losses	(44,000)	(31,000)	(31,000)	(22,000)
Charge-offs	0		0
Balance at the end of the period	1,069,000	1,167,000	1,069,000	1,167,000
NCSC:
Allowance for Loan and Lease Losses [Roll Forward]
Balance at the beginning of the period	6,615,000	6,517,000	5,586,000	6,000,000
Provision (benefit) for credit losses	637,000	276,000	1,666,000	793,000
Charge-offs	0		0
Balance at the end of the period	7,252,000	6,793,000	7,252,000	6,793,000
NCSC: | Electric
Allowance for Loan and Lease Losses [Roll Forward]
Balance at the beginning of the period	4,576,000	4,091,000	3,818,000	3,937,000
Provision (benefit) for credit losses	626,000	290,000	1,384,000	444,000
Charge-offs	0
Balance at the end of the period	5,202,000	4,381,000	5,202,000	4,381,000
NCSC: | Telecom
Allowance for Loan and Lease Losses [Roll Forward]
Balance at the beginning of the period	2,039,000	2,426,000	1,768,000	2,063,000
Provision (benefit) for credit losses	11,000	(14,000)	282,000	349,000
Charge-offs	0
Balance at the end of the period	$ 2,050,000	$ 2,412,000	$ 2,050,000	$ 2,412,000