Loans - Schedule of Loans Outstanding to Members and Unadvanced Commitments by Loan Type and by Member Class (Details) - USD ($) $ in Thousands	Aug. 31, 2026	May 31, 2026	Aug. 31, 2025
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount	$ 38,317,658	$ 38,403,415	$ 37,552,708
Deferred loan origination costs	19,660		16,893
Loans to members	$ 38,337,318	$ 38,422,128	$ 37,569,601
% of Total	100.00%	100.00%
Long-term loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount	$ 34,054,860	$ 33,820,842
% of Total	89.00%	88.00%
Fixed rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount	$ 32,624,863	$ 32,384,884
% of Total	85.00%	84.00%
Variable rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount	$ 1,429,997	$ 1,435,958
% of Total	4.00%	4.00%
Line of credit loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount	$ 4,262,798	$ 4,582,573
% of Total	11.00%	12.00%
Deferred Loan Origination Costs—CFC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Deferred loan origination costs	$ 19,660	$ 18,713
% of Total	0.00%	0.00%
CFC:
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount	$ 36,620,040	$ 36,747,017
Deferred loan origination costs	$ 20,000	$ 19,000
% of Total	95.00%	95.00%
CFC: \| Distribution
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount	$ 30,227,582	$ 30,085,224
% of Total	79.00%	78.00%
CFC: \| Power supply
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount	$ 6,128,758	$ 6,403,396
% of Total	16.00%	17.00%
CFC: \| Statewide and associate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount	$ 263,700	$ 258,397
% of Total	0.00%	0.00%
NCSC:
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount	$ 1,697,618	$ 1,656,398
% of Total	5.00%	5.00%
NCSC: \| Electric
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount	$ 1,111,967	$ 1,073,794
% of Total	3.00%	3.00%
NCSC: \| Telecom
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount	$ 585,651	$ 582,604
% of Total	2.00%	2.00%

Percentage, before allowance for credit loss, of financing receivable to total financing receivable.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph c
-Publisher SEC

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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-Topic 310
-SubTopic 10
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-Section 50
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Amortized cost excluding accrued interest, before allowance for credit loss, of financing receivable. Excludes net investment in lease.

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Amortized cost excluding accrued interest, before allowance for credit loss, fee, and loan in process, of financing receivable. Excludes net investment in lease.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Amount of unamortized loan commitment, origination, and other costs (fees) on financing receivable recognized as adjustment to yield. Excludes financing receivable covered under loss sharing agreement.

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