Loans - Schedule of Unadvanced Commitments (Details) - Unadvanced commitments - USD ($) $ in Thousands	Feb. 28, 2022	May 31, 2021
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	$ 14,161,226	$ 14,357,039
Long-term loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	5,230,253	5,771,813
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	5,230,253	5,771,813
Lines of credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	8,930,973	8,585,226
Parent Company
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	13,291,118	13,519,108
Parent Company | Distribution
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	9,235,009	9,387,070
Parent Company | Power supply
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	3,877,775	3,970,698
Parent Company | Statewide and associate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	178,334	161,340
NCSC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	547,100	551,125
RTFC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	$ 323,008	$ 286,806