Loans and the Allowance for Credit Losses - Recorded Investment in Non-Accrual Loans and TDRs (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Financing Receivable, Impaired [Line Items]
Non-accrual loans	$ 32,898	$ 30,700
TDRs	7,411	8,599
Accruing TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	5,487	5,744
Non-Accrual TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	1,924	2,855
Commercial Real Estate - Land and Construction [Member]
Financing Receivable, Impaired [Line Items]
Non-accrual loans	295
Commercial Real Estate - Improved Property [Member]
Financing Receivable, Impaired [Line Items]
Non-accrual loans	11,726	8,413
TDRs	1,301	2,409
Commercial Real Estate - Improved Property [Member] | Accruing TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	743	880
Commercial Real Estate - Improved Property [Member] | Non-Accrual TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	558	1,529
Commercial and Industrial [Member]
Financing Receivable, Impaired [Line Items]
Non-accrual loans	2,854	3,260
TDRs	192	337
Commercial and Industrial [Member] | Accruing TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	192	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	4,886	4,610
TDRs	922	624
Home Equity [Member] | Accruing TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	486	426
Home Equity [Member] | Non-Accrual TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	436	198
Commercial Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Non-accrual loans	12,021	8,413
TDRs	1,301	2,409
Commercial Real Estate [Member] | Accruing TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	743	880
Commercial Real Estate [Member] | Non-Accrual TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	558	1,529
Residential Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Non-accrual loans	12,813	13,831
TDRs	4,922	5,106
Residential Real Estate [Member] | Accruing TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	4,005	4,185
Residential Real Estate [Member] | Non-Accrual TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	917	921
Consumer [Member]
Financing Receivable, Impaired [Line Items]
Non-accrual loans	324	586
TDRs	74	123
Consumer [Member] | Accruing TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	61	85
Consumer [Member] | Non-Accrual TDRs [Member]
Financing Receivable, Impaired [Line Items]
TDRs	$ 13	$ 38