Loans and Allowance for Loan Losses - Summary of TDR Loans on Accrual and Nonaccrual (Detail) $ in Thousands	3 Months Ended		9 Months Ended			12 Months Ended
	Sep. 30, 2021 USD ($) contract	Sep. 30, 2020 contract	Sep. 30, 2021 USD ($) loan	Sep. 30, 2021 USD ($) contract	Sep. 30, 2020 contract	Dec. 31, 2020 USD ($) loan
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans	2	3	263	4	25	301
Balance of Loans	$ 50,826		$ 50,826	$ 50,826		$ 53,204
TDRs on Accrual Status
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loan			209			248
Balance of Loans	34,723		$ 34,723	34,723		$ 41,095
TDRs on Non-Accrual Status
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loan			54			53
Balance of Loans	16,103		$ 16,103	16,103		$ 12,109
Commercial and Industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans		0	7		1	8
Balance of Loans	14,884		$ 14,884	14,884		$ 12,447
Commercial and Industrial | TDRs on Accrual Status
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loan			1			1
Balance of Loans	3,940		$ 3,940	3,940		$ 5,628
Commercial and Industrial | TDRs on Non-Accrual Status
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loan			6			7
Balance of Loans	10,944		$ 10,944	10,944		$ 6,819
Commercial Real Estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans		0	1		2	2
Balance of Loans	3,520		$ 3,520	3,520		$ 4,001
Commercial Real Estate | TDRs on Accrual Status
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loan			1			1
Balance of Loans	3,520		$ 3,520	3,520		$ 3,521
Commercial Real Estate | TDRs on Non-Accrual Status
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loan			0			1
Balance of Loans	$ 0		$ 0	$ 0		$ 480
Business banking
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans	0	0	10	1	3	12
Balance of Loans	$ 5,150		$ 5,150	$ 5,150		$ 5,193
Business banking | TDRs on Accrual Status
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loan			5			6
Balance of Loans	3,888		$ 3,888	3,888		$ 4,471
Business banking | TDRs on Non-Accrual Status
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loan			5			6
Balance of Loans	$ 1,262		$ 1,262	$ 1,262		$ 722
Residential real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans	0	0	153	1	3	173
Balance of Loans	$ 23,227		$ 23,227	$ 23,227		$ 26,689
Residential real estate | TDRs on Accrual Status
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loan			127			146
Balance of Loans	20,092		$ 20,092	20,092		$ 23,416
Residential real estate | TDRs on Non-Accrual Status
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loan			26			27
Balance of Loans	$ 3,135		$ 3,135	$ 3,135		$ 3,273
Consumer home equity
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans	2	0	89	2	12	103
Balance of Loans	$ 4,024		$ 4,024	$ 4,024		$ 4,845
Consumer home equity | TDRs on Accrual Status
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loan			73			91
Balance of Loans	3,280		$ 3,280	3,280		$ 4,030
Consumer home equity | TDRs on Non-Accrual Status
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loan			16			12
Balance of Loans	744		$ 744	744		$ 815
Other Consumer
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans		3	3		4	3
Balance of Loans	21		$ 21	21		$ 29
Other Consumer | TDRs on Accrual Status
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loan			2			3
Balance of Loans	3		$ 3	3		$ 29
Other Consumer | TDRs on Non-Accrual Status
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loan			1			0
Balance of Loans	$ 18		$ 18	$ 18		$ 0