Loans - Schedule of Unadvanced Commitments (Details) - Unadvanced commitments - USD ($) $ in Thousands	Aug. 31, 2022	May 31, 2022
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	$ 14,174,077	$ 14,111,202
Long-term loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	5,402,527	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	5,402,527	5,357,205
Lines of credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	8,771,550	8,753,997
Parent Company
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	13,344,770	13,249,577
Parent Company | Distribution
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	9,337,695	9,230,197
Parent Company | Power supply
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	3,825,651	3,835,535
Parent Company | Statewide and associate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	181,424	183,845
NCSC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	537,888	551,901
RTFC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	$ 291,419	$ 309,724