Loans - Outstanding Principal Balance and Unadvanced Commitments (Details) - USD ($) $ in Thousands	Feb. 28, 2022	May 31, 2021	Feb. 28, 2021
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount	$ 29,508,363	$ 28,415,107	$ 28,315,671
Deferred loan origination costs	12,018		$ 11,810
Loans to members	$ 29,520,381	$ 28,426,961
% of total loans outstanding	100.00%	100.00%
Fixed rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount	$ 26,452,342	$ 25,514,766
% of total loans outstanding	90.00%	90.00%
Long-term variable rate loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount	$ 720,027	$ 658,579
% of total loans outstanding	2.00%	2.00%
Long-term loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount	$ 27,172,369	$ 26,173,345
% of total loans outstanding	92.00%	92.00%
Lines of credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount	$ 2,335,994	$ 2,241,762
% of total loans outstanding	8.00%	8.00%
Deferred loan origination costs—CFC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Deferred loan origination costs	$ 12,018	$ 11,854
% of total loans outstanding	0.00%	0.00%
Parent Company
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount	$ 28,320,476	$ 27,287,856
Deferred loan origination costs	$ 12,000	$ 12,000
% of total loans outstanding	96.00%	96.00%
Parent Company \| Distribution
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount	$ 23,225,751	$ 22,027,423
% of total loans outstanding	79.00%	78.00%
Parent Company \| Power supply
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount	$ 4,992,073	$ 5,154,312
% of total loans outstanding	17.00%	18.00%
Parent Company \| Statewide and associate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount	$ 102,652	$ 106,121
% of total loans outstanding	0.00%	0.00%
NCSC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount	$ 730,147	$ 706,868
% of total loans outstanding	2.00%	3.00%
RTFC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount	$ 457,740	$ 420,383
% of total loans outstanding	2.00%	1.00%

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

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Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

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Amount, before allowance for credit loss, of financing receivable.

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