Loans and the Allowance for Credit Losses - Recorded Investment in Non-Accrual Loans and TDRs (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Impaired [Line Items]
Non-accrual loans	$ 44,913	$ 30,700
TDRs	6,853	8,599
Accruing TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	5,431	5,744
Non-Accrual TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	1,422	2,855
Commercial Real Estate - Land and Construction [Member]
Financing Receivable, Impaired [Line Items]
Non-accrual loans	580
Commercial Real Estate - Improved Property [Member]
Financing Receivable, Impaired [Line Items]
Non-accrual loans	6,815	8,413
TDRs	1,512	2,409
Commercial Real Estate - Improved Property [Member] | Accruing TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	1,321	880
Commercial Real Estate - Improved Property [Member] | Non-Accrual TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	191	1,529
Commercial and Industrial [Member]
Financing Receivable, Impaired [Line Items]
Non-accrual loans	14,313	3,260
TDRs	191	337
Commercial and Industrial [Member] | Accruing TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	191	168
Commercial and Industrial [Member] | Non-Accrual TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs		169
Home Equity [Member]
Financing Receivable, Impaired [Line Items]
Non-accrual loans	5,903	4,610
TDRs	704	624
Home Equity [Member] | Accruing TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	411	426
Home Equity [Member] | Non-Accrual TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	293	198
Commercial Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Non-accrual loans	7,395	8,413
TDRs	1,512	2,409
Commercial Real Estate [Member] | Accruing TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	1,321	880
Commercial Real Estate [Member] | Non-Accrual TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	191	1,529
Residential Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Non-accrual loans	16,867	13,831
TDRs	4,386	5,106
Residential Real Estate [Member] | Accruing TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	3,477	4,185
Residential Real Estate [Member] | Non-Accrual TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	909	921
Consumer [Member]
Financing Receivable, Impaired [Line Items]
Non-accrual loans	435	586
TDRs	60	123
Consumer [Member] | Accruing TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	31	85
Consumer [Member] | Non-Accrual TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	$ 29	$ 38