Loans and Allowance for Loan Losses - Summary of TDR Loans on Accrual and Nonaccrual (Detail) $ in Thousands	3 Months Ended			12 Months Ended
	Mar. 31, 2021 USD ($) loans	Mar. 31, 2021 USD ($) contract	Mar. 31, 2020 contract	Dec. 31, 2020 USD ($) loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans	286	1	10	301
Balance of Loans	$ 52,904	$ 52,904		$ 53,204
Special Mention | Contractual Obligation Modification | TDRs on Accrual Status
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loans	229			248
Balance of Loans	$ 39,367	39,367		$ 41,095
Special Mention | Contractual Obligation Modification | TDRs on Nonaccrual Status
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loans	57			53
Balance of Loans	$ 13,537	13,537		$ 12,109
Commercial and industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loans	7			8
Balance of Loans	$ 13,224	13,224		$ 12,447
Commercial and industrial | Special Mention | Contractual Obligation Modification | TDRs on Accrual Status
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loans	1			1
Balance of Loans	$ 5,641	5,641		$ 5,628
Commercial and industrial | Special Mention | Contractual Obligation Modification | TDRs on Nonaccrual Status
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loans	6			7
Balance of Loans	$ 7,583	7,583		$ 6,819
Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loans	2			2
Balance of Loans	$ 3,988	3,988		$ 4,001
Commercial real estate | Special Mention | Contractual Obligation Modification | TDRs on Accrual Status
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loans	1			1
Balance of Loans	$ 3,520	3,520		$ 3,521
Commercial real estate | Special Mention | Contractual Obligation Modification | TDRs on Nonaccrual Status
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loans	1			1
Balance of Loans	$ 468	$ 468		$ 480
Business banking
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans	11	0	1	12
Balance of Loans	$ 5,083	$ 5,083		$ 5,193
Business banking | Special Mention | Contractual Obligation Modification | TDRs on Accrual Status
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loans	5			6
Balance of Loans	$ 4,008	4,008		$ 4,471
Business banking | Special Mention | Contractual Obligation Modification | TDRs on Nonaccrual Status
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loans	6			6
Balance of Loans	$ 1,075	$ 1,075		$ 722
Residential real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans	170	1	8	173
Balance of Loans	$ 26,122	$ 26,122		$ 26,689
Residential real estate | Special Mention | Contractual Obligation Modification | TDRs on Accrual Status
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loans	140			146
Balance of Loans	$ 22,565	22,565		$ 23,416
Residential real estate | Special Mention | Contractual Obligation Modification | TDRs on Nonaccrual Status
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loans	30			27
Balance of Loans	$ 3,557	$ 3,557		$ 3,273
Consumer home equity
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans	93	0	1	103
Balance of Loans	$ 4,461	$ 4,461		$ 4,845
Consumer home equity | Special Mention | Contractual Obligation Modification | TDRs on Accrual Status
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loans	79			91
Balance of Loans	$ 3,607	3,607		$ 4,030
Consumer home equity | Special Mention | Contractual Obligation Modification | TDRs on Nonaccrual Status
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loans	14			12
Balance of Loans	$ 854	854		$ 815
Other consumer
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loans	3			3
Balance of Loans	$ 26	26		$ 29
Other consumer | Special Mention | Contractual Obligation Modification | TDRs on Accrual Status
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loans	3			3
Balance of Loans	$ 26	26		$ 29
Other consumer | Special Mention | Contractual Obligation Modification | TDRs on Nonaccrual Status
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loans	0			0
Balance of Loans	$ 0	$ 0		$ 0