Loans and the Allowance for Credit Losses - Recorded Investment in Non-Accrual Loans and TDRs (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Financing Receivable, Impaired [Line Items]
Non-accrual loans	$ 36,880	$ 44,913
TDRs	5,755	6,853
Accruing TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	3,927	5,431
Non-Accrual TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	1,828	1,422
Commercial and Industrial [Member]
Financing Receivable, Impaired [Line Items]
Non-accrual loans	3,302	14,313
TDRs	111	191
Commercial and Industrial [Member] | Accruing TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	111	191
Home Equity [Member]
Financing Receivable, Impaired [Line Items]
Non-accrual loans	5,345	5,903
TDRs	663	704
Home Equity [Member] | Accruing TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	363	411
Home Equity [Member] | Non-Accrual TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	300	293
Commercial Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Non-accrual loans	9,963	7,395
TDRs	820	1,512
Commercial Real Estate [Member] | Accruing TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	655	1,321
Commercial Real Estate [Member] | Non-Accrual TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	165	191
Commercial Real Estate [Member] | Commercial Real Estate - Land and Construction [Member]
Financing Receivable, Impaired [Line Items]
Non-accrual loans	469	580
Commercial Real Estate [Member] | Commercial Real Estate - Improved Property [Member]
Financing Receivable, Impaired [Line Items]
Non-accrual loans	9,494	6,815
TDRs	820	1,512
Commercial Real Estate [Member] | Commercial Real Estate - Improved Property [Member] | Accruing TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	655	1,321
Commercial Real Estate [Member] | Commercial Real Estate - Improved Property [Member] | Non-Accrual TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	165	191
Residential Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Non-accrual loans	17,925	16,867
TDRs	4,133	4,386
Residential Real Estate [Member] | Accruing TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	2,779	3,477
Residential Real Estate [Member] | Non-Accrual TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	1,354	909
Consumer [Member]
Financing Receivable, Impaired [Line Items]
Non-accrual loans	345	435
TDRs	28	60
Consumer [Member] | Accruing TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	19	31
Consumer [Member] | Non-Accrual TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	$ 9	$ 29