Loans - Unadvanced loan commitments Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	May 31, 2026	May 31, 2025
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unadvanced line of credit commitments as percentage of unadvanced loan commitments	58.00%
Unadvanced line of credit commitments, term	5 years
Unadvanced commitments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total unadvanced commitments	$ 20,013,899	$ 18,219,009
Commitments to extend credit subject to material adverse change clause
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total unadvanced commitments	16,288,000	14,629,000
Unadvanced commitments not subject to material adverse change clauses
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total unadvanced commitments	3,725,868	3,590,000
Variable rate | Unadvanced commitments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total unadvanced commitments	$ 8,397,685	7,471,266
Financing receivable, loan commitment, term	5 years
Long-term loans | Unadvanced commitments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total unadvanced commitments	$ 8,397,685	$ 7,471,266