Allowance for Loan Losses - Summary of Pre and Post-modification Balances for Loans Newly Designated as TDRs (Detail) $ in Thousands		3 Months Ended	9 Months Ended
		Sep. 30, 2020 USD ($) Loan	Sep. 30, 2020 USD ($) Loan	Sep. 30, 2019 USD ($) Loan
Commercial Mortgages (Non-Owner Occupied)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | Loan	[1]	1	1
Pre-Modification Outstanding Recorded Investment	[1]	$ 220	$ 220
Post-Modification Outstanding Recorded Investment	[1]	$ 220	$ 220
Residential First Mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | Loan	[2]		1
Pre-Modification Outstanding Recorded Investment	[2]		$ 391
Post-Modification Outstanding Recorded Investment	[2]		$ 391
Commercial Mortgages (Owner Occupied)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | Loan	[1]			1
Pre-Modification Outstanding Recorded Investment	[1]			$ 48
Post-Modification Outstanding Recorded Investment	[1]			$ 52

[1] Modification was an extension of the loan term.

[2] Modification was an interest payment deferral.