Loans - Schedule of unadvanced commitments (Details) - Unadvanced commitments - USD ($) $ in Thousands	May 31, 2026	May 31, 2025
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total unadvanced commitments	$ 20,013,899	$ 18,219,009
Long-term loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total unadvanced commitments	8,397,685	7,471,266
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	8,397,685	7,471,266
Lines of credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total unadvanced commitments	11,616,214	10,747,743
CFC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total unadvanced commitments	19,290,518	17,261,682
CFC | Distribution
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total unadvanced commitments	13,576,857	11,948,516
CFC | Power supply
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total unadvanced commitments	5,501,553	5,097,398
CFC | Statewide and associate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total unadvanced commitments	212,108	215,768
NCSC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total unadvanced commitments	723,381	957,327
NCSC | Electric
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total unadvanced commitments	359,203	520,312
NCSC | Telecom
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total unadvanced commitments	$ 364,178	$ 437,015