Loans - Borrower risk ratings Narrative (Details) $ in Thousands	12 Months Ended
	May 31, 2026 USD ($) borrower hurricane loan	May 31, 2025 USD ($) loan borrower
Accounts, Notes, Loans and Financing Receivable [Line Items]
Term of loans	35 years
Loans originated, five years before reporting period	$ 19,273,122	$ 18,536,659
Financing receivable, percent originated, more than five years before current fiscal year	50.00%	50.00%
Financing Receivable, before Allowance for Credit Loss, to Total, Percent	100.00%	100.00%
Long-term financing receivable, before allowance for credit loss, average remaining maturity	19 years	19 years
Loans to members	$ 38,422,128	$ 37,079,978
Number of active borrowers | borrower	903	899
Number of successive hurricanes | hurricane	2
CFC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, before Allowance for Credit Loss, to Total, Percent	95.00%	95.00%
CFC | Distribution
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, before Allowance for Credit Loss, to Total, Percent	78.00%	79.00%
Criticized
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans originated, five years before reporting period	$ 36,448	$ 52,776
Loans to members	$ 207,000	$ 219,000
Financing receivable, before allowance for credit loss, percentage	1.00%	1.00%
Increase (decrease) in finance receivables	$ (12,000)
Special mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans originated, five years before reporting period	28,948	$ 26,677
Loans to members	199,000	193,000
Increase (decrease) in finance receivables	(18,000)
Special mention | CFC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans originated, five years before reporting period	28,948	26,677
Special mention | CFC | Distribution
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans originated, five years before reporting period	19,224	15,693
Special mention | CFC | CFC electric distribution borrower | Distribution
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans to members	$ 189,000	$ 181,000
Number of active borrowers | borrower	1	1
Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans originated, five years before reporting period	$ 0	$ 0
Loans to members	0	0
Doubtful
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans originated, five years before reporting period	7,500	26,099
Loans to members	8,000	$ 26,000
Increase (decrease) in finance receivables	$ 6,000
Number of loans outstanding | loan	1	1
Long-term loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, before Allowance for Credit Loss, to Total, Percent	88.00%	88.00%