Loans - Nonperforming Loans (Details) $ in Thousands	Aug. 31, 2023 USD ($) borrower	May 31, 2023 USD ($) borrower	Aug. 31, 2022 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans to members	$ 33,096,646	$ 32,532,086	$ 30,687,331
% of Total Loans Outstanding	100.00%	100.00%
Nonperforming financial instruments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Borrowers | borrower	1	2
Loans to members	$ 84,987	$ 89,334
% of Total Loans Outstanding	0.26%	0.27%
Parent Company
Accounts, Notes, Loans and Financing Receivable [Line Items]
% of Total Loans Outstanding	96.00%	96.00%
Parent Company | Power supply
Accounts, Notes, Loans and Financing Receivable [Line Items]
% of Total Loans Outstanding	17.00%	17.00%
Parent Company | Power supply | Nonperforming financial instruments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Borrowers | borrower	1	2
Loans to members	$ 84,987	$ 89,334
% of Total Loans Outstanding	0.26%	0.27%