Loans - Schedule of unadvanced commitments (Details) - Unadvanced commitments - USD ($) $ in Thousands	May 31, 2023	May 31, 2022
Accounts, Notes, Loans and Financing Receivable [Line Items]
Available balance	$ 14,788,561	$ 14,111,202
Long-term loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Available balance	5,669,634	5,357,205
Fixed rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Available balance	0	0
Variable rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Available balance	5,669,634	5,357,205
Lines of credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Available balance	9,118,927	8,753,997
CFC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Available balance	13,843,990	13,249,577
CFC | Distribution
Accounts, Notes, Loans and Financing Receivable [Line Items]
Available balance	9,673,712	9,230,197
CFC | Power supply
Accounts, Notes, Loans and Financing Receivable [Line Items]
Available balance	3,995,128	3,835,535
CFC | Statewide and associate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Available balance	175,150	183,845
NCSC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Available balance	604,436	551,901
RTFC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Available balance	$ 340,135	$ 309,724