Allowance for Loan Losses - Troubled Debt Restructurings (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) SecurityLoan	Dec. 31, 2018 USD ($) SecurityLoan	Dec. 31, 2017 USD ($) SecurityLoan
Financing Receivable, Modifications [Line Items]
Number | SecurityLoan	18	17	26
Pre-mod outstanding principal balance	$ 2,851	$ 3,768	$ 7,111
Post-mod outstanding principal balance	2,829	3,755	6,978
Financial impact due to TDR taken as additional provision	$ 59	$ 781	$ 133
Number that defaulted during the period | SecurityLoan	1	6	5
Recorded investment of TDRs that defaulted during the period	$ 7	$ 565	$ 624
Financial impact due to the default of previous TDR taken as charge- offs or additional provisions	$ 0		$ (5)
Mortgage Loans on Real Estate
Financing Receivable, Modifications [Line Items]
Number | SecurityLoan	5	8	9
Pre-mod outstanding principal balance	$ 719	$ 1,938	$ 4,660
Post-mod outstanding principal balance	729	1,935	4,634
Financial impact due to TDR taken as additional provision	$ 30	$ 491	$ 58
Number that defaulted during the period | SecurityLoan	0	1	3
Recorded investment of TDRs that defaulted during the period	$ 0	$ 169	$ 442
Financial impact due to the default of previous TDR taken as charge- offs or additional provisions	$ 0
Consumer
Financing Receivable, Modifications [Line Items]
Number | SecurityLoan	3	3	5
Pre-mod outstanding principal balance	$ 214	$ 732	$ 453
Post-mod outstanding principal balance	215	737	453
Financial impact due to TDR taken as additional provision	$ 29	$ (35)	$ 38
Number that defaulted during the period | SecurityLoan	0	2	2
Recorded investment of TDRs that defaulted during the period	$ 0	$ 248	$ 182
Financial impact due to the default of previous TDR taken as charge- offs or additional provisions	$ 0
Commercial
Financing Receivable, Modifications [Line Items]
Number | SecurityLoan	10	6	11
Pre-mod outstanding principal balance	$ 1,918	$ 1,098	$ 1,854
Post-mod outstanding principal balance	1,885	1,083	1,747
Financial impact due to TDR taken as additional provision	$ 0	$ 325	$ 37
Number that defaulted during the period | SecurityLoan	1	3
Recorded investment of TDRs that defaulted during the period	$ 7	$ 148
Financial impact due to the default of previous TDR taken as charge- offs or additional provisions	$ 0
Construction
Financing Receivable, Modifications [Line Items]
Number | SecurityLoan	0		1
Pre-mod outstanding principal balance	$ 0		$ 144
Post-mod outstanding principal balance	0		$ 144
Financial impact due to TDR taken as additional provision	$ 0
Number that defaulted during the period | SecurityLoan	0
Recorded investment of TDRs that defaulted during the period	$ 0
Financial impact due to the default of previous TDR taken as charge- offs or additional provisions	$ 0
Residential 1-4 family | Mortgage Loans on Real Estate
Financing Receivable, Modifications [Line Items]
Number | SecurityLoan	3	1	1
Pre-mod outstanding principal balance	$ 659	$ 156	$ 939
Post-mod outstanding principal balance	662	$ 156	939
Financial impact due to TDR taken as additional provision	$ 30		$ 169
Number that defaulted during the period | SecurityLoan	0		2
Recorded investment of TDRs that defaulted during the period	$ 0		$ 223
Financial impact due to the default of previous TDR taken as charge- offs or additional provisions	$ 0
Home equity loans | Consumer
Financing Receivable, Modifications [Line Items]
Number | SecurityLoan	2	2	1
Pre-mod outstanding principal balance	$ 149	$ 599	$ 252
Post-mod outstanding principal balance	147	599	$ 252
Financial impact due to TDR taken as additional provision	$ 29	$ (35)
Number that defaulted during the period | SecurityLoan	0		1
Recorded investment of TDRs that defaulted during the period	$ 0		$ 55
Financial impact due to the default of previous TDR taken as charge- offs or additional provisions	$ 0
Other | Consumer
Financing Receivable, Modifications [Line Items]
Number | SecurityLoan	0		1
Pre-mod outstanding principal balance	$ 0		$ 14
Post-mod outstanding principal balance	0		14
Financial impact due to TDR taken as additional provision	$ 0		$ 11
Number that defaulted during the period | SecurityLoan	0
Recorded investment of TDRs that defaulted during the period	$ 0
Financial impact due to the default of previous TDR taken as charge- offs or additional provisions	$ 0
Commercial | Mortgage Loans on Real Estate
Financing Receivable, Modifications [Line Items]
Number | SecurityLoan	2	7	8
Pre-mod outstanding principal balance	$ 60	$ 1,782	$ 3,721
Post-mod outstanding principal balance	67	1,779	3,695
Financial impact due to TDR taken as additional provision	$ 0	$ 491	$ (111)
Number that defaulted during the period | SecurityLoan	0	1	1
Recorded investment of TDRs that defaulted during the period	$ 0	$ 169	$ 219
Financial impact due to the default of previous TDR taken as charge- offs or additional provisions	$ 0
Home equity lines of credit | Consumer
Financing Receivable, Modifications [Line Items]
Number | SecurityLoan	1	1	3
Pre-mod outstanding principal balance	$ 65	$ 133	$ 187
Post-mod outstanding principal balance	68	$ 138	187
Financial impact due to TDR taken as additional provision	$ 0		$ 27
Number that defaulted during the period | SecurityLoan	0	2	1
Recorded investment of TDRs that defaulted during the period	$ 0	$ 248	$ 127
Financial impact due to the default of previous TDR taken as charge- offs or additional provisions	$ 0		$ (5)
Residential | Construction
Financing Receivable, Modifications [Line Items]
Number | SecurityLoan	0		1
Pre-mod outstanding principal balance	$ 0		$ 144
Post-mod outstanding principal balance	0		$ 144
Financial impact due to TDR taken as additional provision	$ 0
Number that defaulted during the period | SecurityLoan	0
Recorded investment of TDRs that defaulted during the period	$ 0
Financial impact due to the default of previous TDR taken as charge- offs or additional provisions	$ 0