Loans - Schedule of Unadvanced Commitments (Details) - Unadvanced commitments - USD ($) $ in Thousands	Feb. 28, 2023	May 31, 2022
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	$ 14,320,278	$ 14,111,202
Long-term loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	5,643,107	5,357,205
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	5,643,107	5,357,205
Lines of credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	8,677,171	8,753,997
Parent Company
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	13,369,053	13,249,577
Parent Company | Distribution
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	9,332,479	9,230,197
Parent Company | Power supply
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	3,884,174	3,835,535
Parent Company | Statewide and associate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	152,400	183,845
NCSC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	606,121	551,901
RTFC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	$ 345,104	$ 309,724