Loans and Allowance for Loan Losses - Summary of the Modifications Which Occurred During the Periods and the Change in the Recorded Investment Subsequent to the Modifications Occurring (Detail) $ in Thousands	3 Months Ended		9 Months Ended				12 Months Ended
	Sep. 30, 2021 USD ($) contract	Sep. 30, 2020 USD ($) contract	Sep. 30, 2021 loan	Sep. 30, 2021 contract	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($) contract	Dec. 31, 2020 loan
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts	2	3	263	4		25	301
Pre- Modification Outstanding Recorded Investment	$ 200	$ 35			$ 957	$ 2,796
Post-Modification Modification Outstanding Recorded Investment	$ 200	$ 35			957	$ 2,800
Commercial and Industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts		0	7			1	8
Pre- Modification Outstanding Recorded Investment		$ 0				$ 140
Post-Modification Modification Outstanding Recorded Investment		$ 0				$ 140
Commercial Real Estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts		0	1			2	2
Pre- Modification Outstanding Recorded Investment		$ 0				$ 632
Post-Modification Modification Outstanding Recorded Investment		$ 0				$ 632
Business banking
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts	0	0	10	1		3	12
Pre- Modification Outstanding Recorded Investment	$ 0	$ 0			462	$ 1,040
Post-Modification Modification Outstanding Recorded Investment	$ 0	$ 0			462	$ 1,040
Residential real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts	0	0	153	1		3	173
Pre- Modification Outstanding Recorded Investment	$ 0	$ 0			295	$ 399
Post-Modification Modification Outstanding Recorded Investment	$ 0	$ 0			295	$ 399
Consumer home equity
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts	2	0	89	2		12	103
Pre- Modification Outstanding Recorded Investment	$ 200	$ 0			200	$ 527
Post-Modification Modification Outstanding Recorded Investment	$ 200	$ 0			$ 200	$ 531
Other Consumer
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts		3	3			4	3
Pre- Modification Outstanding Recorded Investment		$ 35				$ 58
Post-Modification Modification Outstanding Recorded Investment		$ 35				$ 58