Loans - Schedule of Unadvanced Commitments (Details) - Unadvanced Commitments - USD ($) $ in Thousands	Aug. 31, 2026	May 31, 2026
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total unadvanced commitments	$ 20,935,075	$ 20,013,899
Long-term loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total unadvanced commitments	9,040,261	8,397,685
Fixed rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total unadvanced commitments	0	0
Variable rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total unadvanced commitments	9,040,261	8,397,685
Line of credit loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total unadvanced commitments	11,894,814	11,616,214
CFC:
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total unadvanced commitments	20,167,489	19,290,518
CFC: \| Distribution
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total unadvanced commitments	14,177,129	13,576,857
CFC: \| Power supply
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total unadvanced commitments	5,788,652	5,501,553
CFC: \| Statewide and associate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total unadvanced commitments	201,708	212,108
NCSC:
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total unadvanced commitments	767,586	723,381
NCSC: \| Electric
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total unadvanced commitments	399,309	359,203
NCSC: \| Telecom
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total unadvanced commitments	$ 368,277	$ 364,178