Loans - Schedule of Unadvanced Commitments (Details) - Unadvanced commitments - USD ($) $ in Thousands	Aug. 31, 2023	May 31, 2023
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	$ 14,964,921	$ 14,788,561
Long-term loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	5,796,421	5,669,634
Fixed rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	0	0
Variable rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	5,796,421	5,669,634
Lines of credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	9,168,500	9,118,927
CFC:
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	13,956,106	13,843,990
CFC: | Distribution
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	9,803,363	9,673,712
CFC: | Power supply
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	3,995,643	3,995,128
CFC: | Statewide and associate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	157,100	175,150
NCSC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	633,895	604,436
RTFC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	$ 374,920	$ 340,135