Loans - Schedule of Unadvanced Commitments (Details) - Unadvanced commitments - USD ($) $ in Thousands	Nov. 30, 2021	May 31, 2021
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	$ 14,678,351	$ 14,357,039
Long-term loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	5,663,645	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	5,663,645	5,771,813
Lines of credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	9,014,706	8,585,226
Parent Company
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	13,743,795	13,519,108
Parent Company | Distribution
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	9,659,640	9,387,070
Parent Company | Power supply
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	3,904,003	3,970,698
Parent Company | Statewide and associate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	180,152	161,340
NCSC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	596,904	551,125
RTFC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced commitments	$ 337,652	$ 286,806