Loans and allowance for loan losses - Summary of Payment Status of Loans Modified (Detail) - USD ($) $ in Thousands	Sep. 30, 2025	Sep. 30, 2024
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	$ 1,255,000	$ 1,247,000
Guaranteed by Government-Related Entities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Modified past due 30-89 days	37,000	39,000
Modified past due 90 days or more	41,000	34,000
Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	891,000	986,000
30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	221,000	211,000
Accruing Loans Past Due 90 Days or More
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	143,000	50,000
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	295,000	317,000
Commercial and industrial | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	137,000	308,000
Commercial and industrial | 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	60,000	5,000
Commercial and industrial | Accruing Loans Past Due 90 Days or More
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	98,000	4,000
Commercial Real Estate | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	447,000	515,000
Commercial Real Estate | Commercial | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	328,000	422,000
Commercial Real Estate | Commercial | 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	118,000	84,000
Commercial Real Estate | Commercial | Accruing Loans Past Due 90 Days or More
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	1,000	9,000
Commercial Real Estate | Residential builder and developer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	0	14,000
Commercial Real Estate | Residential builder and developer | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	0	13,000
Commercial Real Estate | Residential builder and developer | 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	0	1,000
Commercial Real Estate | Residential builder and developer | Accruing Loans Past Due 90 Days or More
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	0	0
Commercial Real Estate | Other commercial construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	337,000	215,000
Commercial Real Estate | Other commercial construction | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	337,000	141,000
Commercial Real Estate | Other commercial construction | 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	0	74,000
Commercial Real Estate | Other commercial construction | Accruing Loans Past Due 90 Days or More
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	0	0
Residential Real Estate | Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	163,000	183,000
Residential Real Estate | Residential | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	76,000	99,000
Residential Real Estate | Residential | 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	43,000	47,000
Residential Real Estate | Residential | Accruing Loans Past Due 90 Days or More
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	44,000	37,000
Consumer | Home equity lines and loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	1,000	2,000
Consumer | Home equity lines and loans | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	1,000	2,000
Consumer | Home equity lines and loans | 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	0	0
Consumer | Home equity lines and loans | Accruing Loans Past Due 90 Days or More
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	0	0
Consumer | Recreational finance
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	1,000	1,000
Consumer | Recreational finance | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	1,000	1,000
Consumer | Recreational finance | 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	0	0
Consumer | Recreational finance | Accruing Loans Past Due 90 Days or More
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	0	0
Consumer | Automobile
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	1,000	0
Consumer | Automobile | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	1,000	0
Consumer | Automobile | 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	0	0
Consumer | Automobile | Accruing Loans Past Due 90 Days or More
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	0	0
Consumer | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	10,000	0
Consumer | Other | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	10,000	0
Consumer | Other | 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	0	0
Consumer | Other | Accruing Loans Past Due 90 Days or More
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost	$ 0	$ 0