Allowance for Loan Losses - Performing and Nonperforming Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Financing Receivable, Recorded Investment [Line Items]
Performing	$ 388,369	$ 407,051
Total	388,369	407,051
Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing	361,979	382,720
Total	361,979	382,720
Real Estate Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing	13,935	10,565
Total	13,935	10,565
Consumer and Other [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing	12,455	13,766
Total	$ 12,455	$ 13,766