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				12 Months Ended
	Mar. 31, 2021 loans	Mar. 31, 2021 contract	Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($) contract	Dec. 31, 2020 loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts	286	1		10	301
Pre- Modification Outstanding Recorded Investment			$ 295	$ 682
Post-Modification Modification Outstanding Recorded Investment			295	$ 687
Business banking
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts	11	0		1	12
Pre- Modification Outstanding Recorded Investment			0	$ 244
Post-Modification Modification Outstanding Recorded Investment			0	$ 244
Residential real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts	170	1		8	173
Pre- Modification Outstanding Recorded Investment			295	$ 414
Post-Modification Modification Outstanding Recorded Investment			295	$ 419
Consumer home equity
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts	93	0		1	103
Pre- Modification Outstanding Recorded Investment			0	$ 24
Post-Modification Modification Outstanding Recorded Investment			$ 0	$ 24