Loans Receivable and Related Allowance for Loan Losses (5) (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Sep. 30, 2020
Financing Receivable, Impaired [Line Items]
Total non-accrual loans	$ 22,281	$ 16,730
Residential Mortgage [Member]
Financing Receivable, Impaired [Line Items]
Total non-accrual loans	1,608	2,036
Commercial Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Total non-accrual loans	20,350	14,414
Consumer - Home Equity Lines of Credit [Member]
Financing Receivable, Impaired [Line Items]
Total non-accrual loans	24	26
Consumer - Second Mortgages Receivable [Member]
Financing Receivable, Impaired [Line Items]
Total non-accrual loans	$ 299	$ 254