Allowance for Loan Losses - Performing and Nonperforming Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Financing Receivable, Recorded Investment [Line Items]
Performing	$ 369,625	$ 382,911
Total	369,625	382,911
Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing	345,374	354,987
Total	345,374	354,987
Real Estate Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing	13,467	15,948
Total	13,467	15,948
Consumer and Other [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing	10,784	11,976
Total	$ 10,784	$ 11,976