Allowance for Loan Losses - Troubled Debt Restructurings (Detail) $ in Thousands	12 Months Ended
Dec. 31, 2018 USD ($) SecurityLoan	Dec. 31, 2017 USD ($) SecurityLoan	Dec. 31, 2016 USD ($) SecurityLoan
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan	17	26	24
Pre-mod outstanding principal balance	$ 3,768	$ 7,111	$ 1,989
Post-mod outstanding principal balance	3,755	6,978	2,035
Financial impact due to TDR taken as additional provision	$ 781	$ 133	$ 326
Number that defaulted during the period \| SecurityLoan	6	5	3
Recorded investment of TDRs that defaulted during the period	$ 565	$ 624	$ 330
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions		$ (5)
Mortgage Loans on Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan	8	9	8
Pre-mod outstanding principal balance	$ 1,938	$ 4,660	$ 1,073
Post-mod outstanding principal balance	1,935	4,634	1,117
Financial impact due to TDR taken as additional provision	$ 491	$ 58	$ 96
Number that defaulted during the period \| SecurityLoan	1	3	2
Recorded investment of TDRs that defaulted during the period	$ 169	$ 442	$ 101
Consumer [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan	3	5	12
Pre-mod outstanding principal balance	$ 732	$ 453	$ 839
Post-mod outstanding principal balance	737	453	841
Financial impact due to TDR taken as additional provision	$ (35)	$ 38	$ 207
Number that defaulted during the period \| SecurityLoan	2	2	1
Recorded investment of TDRs that defaulted during the period	$ 248	$ 182	$ 229
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions		$ (5)
Commercial Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan	6	11	4
Pre-mod outstanding principal balance	$ 1,098	$ 1,854	$ 77
Post-mod outstanding principal balance	1,083	1,747	77
Financial impact due to TDR taken as additional provision	$ 325	$ 37	$ 23
Number that defaulted during the period \| SecurityLoan	3
Recorded investment of TDRs that defaulted during the period	$ 148
Construction [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan		1
Pre-mod outstanding principal balance		$ 144
Post-mod outstanding principal balance		$ 144
Residential 1-4 Family [Member] \| Mortgage Loans on Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan	1	1	3
Pre-mod outstanding principal balance	$ 156	$ 939	$ 650
Post-mod outstanding principal balance	$ 156	939	656
Financial impact due to TDR taken as additional provision		$ 169	$ 50
Number that defaulted during the period \| SecurityLoan		2	2
Recorded investment of TDRs that defaulted during the period		$ 223	$ 101
Home Equity Loans [Member] \| Consumer [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan	2	1	1
Pre-mod outstanding principal balance	$ 599	$ 252	$ 105
Post-mod outstanding principal balance	599	$ 252	$ 105
Financial impact due to TDR taken as additional provision	$ (35)
Number that defaulted during the period \| SecurityLoan		1
Recorded investment of TDRs that defaulted during the period		$ 55
Other Consumer [Member] \| Consumer [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan		1	2
Pre-mod outstanding principal balance		$ 14	$ 27
Post-mod outstanding principal balance		14	27
Financial impact due to TDR taken as additional provision		$ 11	$ 2
Commercial [Member] \| Mortgage Loans on Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan	7	8	5
Pre-mod outstanding principal balance	$ 1,782	$ 3,721	$ 423
Post-mod outstanding principal balance	1,779	3,695	461
Financial impact due to TDR taken as additional provision	$ 491	$ (111)	$ 46
Number that defaulted during the period \| SecurityLoan	1	1
Recorded investment of TDRs that defaulted during the period	$ 169	$ 219
Home Equity Lines of Credit [Member] \| Consumer [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan	1	3	9
Pre-mod outstanding principal balance	$ 133	$ 187	$ 707
Post-mod outstanding principal balance	$ 138	187	709
Financial impact due to TDR taken as additional provision		$ 27	$ 205
Number that defaulted during the period \| SecurityLoan	2	1	1
Recorded investment of TDRs that defaulted during the period	$ 248	$ 127	$ 229
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions		$ (5)
Residential [Member] \| Construction [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan		1
Pre-mod outstanding principal balance		$ 144
Post-mod outstanding principal balance		$ 144

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Number of financing receivables that have been modified by troubled debt restructurings.

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Number of financing receivables that have been modified by troubled debt restructurings within the previous 12 months and for which there was a payment default.

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