Allowance for Credit Losses (TDRs Narrative) (Detail)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021 USD ($) contract	Sep. 30, 2020 USD ($) contract	Sep. 30, 2021 USD ($) contract	Sep. 30, 2020 USD ($) contract	Sep. 30, 2021 USD ($) contract	Sep. 30, 2020 USD ($) contract
Financing Receivable, Allowance for Credit Loss [Line Items]
TDRs, count | contract	9	36	45	103	66	152
Foreclosed residential real estate properties	$ 2,500,000		$ 2,500,000		$ 2,500,000
TDRs, balance	$ 559,000	$ 7,596,000	$ 6,884,000	$ 40,410,000	12,345,000	$ 47,173,000
Weighted average extension term	105 months	26 months	108 months	14 months
Weighted average decrease in stated interest rate	1.29%	1.16%	1.51%	1.43%
Interest-only payment terms		5 months	6 months	13 months
Loan forgiveness	$ 0	$ 0	$ 0	$ 453,000
Financing Receivable
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans modified in TDRs	48,500,000		$ 48,500,000		48,500,000
TDRs, count | contract			254
Allowance for loan losses related to impaired loans	2,300,000		$ 2,300,000		2,300,000
Residential Real Estate | Financing Receivable
Financing Receivable, Allowance for Credit Loss [Line Items]
Recorded investment, foreclosure proceedings in process	$ 14,300,000	$ 17,700,000	$ 14,300,000	$ 17,700,000	$ 14,300,000	$ 17,700,000