Allowance for Loan Losses - Troubled Debt Restructurings (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($) SecurityLoan Investmentsecurities	Sep. 30, 2018 USD ($) SecurityLoan Investmentsecurities	Sep. 30, 2019 USD ($) SecurityLoan Branches Investmentsecurities	Sep. 30, 2018 USD ($) SecurityLoan
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number | SecurityLoan	4,000	7	18	13
Pre-mod outstanding principal balance	$ 706,000	$ 2,007,000	$ 2,851,000	$ 3,094,000
Post-mod outstanding principal balance	715,000	2,005,000	2,829,000	3,101,000
Financial impact due to TDR taken as additional provision	$ 28,000	$ (308,000)	$ 59,000	$ (265,000)
Number that defaulted during the period		1,000	1	6
Recorded investment of TDRs that defaulted during the period		$ 128,000	$ 7,000	$ 637,000
Financial impact due to the default of previous TDR taken as charge- offs or additional provisions		$ 0		$ (2,000)
Mortgage Loans on Real Estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number	4,000	4	5	6
Pre-mod outstanding principal balance	$ 556,000	$ 1,326,000	$ 719,000	$ 1,743,000
Post-mod outstanding principal balance	567,000	1,324,000	729,000	1,741,000
Financial impact due to TDR taken as additional provision	$ 30,000	$ (308,000)	$ 30,000	$ (262,000)
Number that defaulted during the period				1
Recorded investment of TDRs that defaulted during the period				$ 169,000
Consumer
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number	0	1	3	3
Pre-mod outstanding principal balance	$ 0	$ 478,000	$ 214,000	$ 732,000
Post-mod outstanding principal balance	0	$ 478,000	215,000	$ 737,000
Financial impact due to TDR taken as additional provision	$ 0		$ 29,000
Number that defaulted during the period		1,000		1
Recorded investment of TDRs that defaulted during the period		$ 128,000		$ 128,000
Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number | SecurityLoan	4	2	10	4
Pre-mod outstanding principal balance	$ 150,000	$ 203,000	$ 1,918,000	$ 619,000
Post-mod outstanding principal balance	148,000	203,000	1,885,000	623,000
Financial impact due to TDR taken as additional provision	$ (2,000)	$ 0	$ 0	$ (3,000)
Number that defaulted during the period		0	1	4
Recorded investment of TDRs that defaulted during the period		$ 0	$ 7,000	$ 340,000
Financial impact due to the default of previous TDR taken as charge- offs or additional provisions		$ 0		$ (2,000)
Residential 1-4 family | Mortgage Loans on Real Estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number	2,000		3
Pre-mod outstanding principal balance	$ 496,000		$ 659,000
Post-mod outstanding principal balance	500,000		662,000
Financial impact due to TDR taken as additional provision	$ 30,000		$ 30,000
Home equity loans | Consumer
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number		1	2	2
Pre-mod outstanding principal balance		$ 478,000	$ 149,000	$ 599,000
Post-mod outstanding principal balance		$ 478,000	147,000	$ 599,000
Financial impact due to TDR taken as additional provision			$ 29,000
Commercial loans | Mortgage Loans on Real Estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number	2,000	4	2	6
Pre-mod outstanding principal balance	$ 60,000	$ 1,326,000	$ 60,000	$ 1,743,000
Post-mod outstanding principal balance	$ 67,000	1,324,000	$ 67,000	1,741,000
Financial impact due to TDR taken as additional provision		$ (308,000)		$ (262,000)
Number that defaulted during the period				1
Recorded investment of TDRs that defaulted during the period				$ 169,000
Home equity lines of credit | Consumer
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number | SecurityLoan			1	1
Pre-mod outstanding principal balance			$ 65,000	$ 133,000
Post-mod outstanding principal balance			$ 68,000	$ 138,000
Number that defaulted during the period		1,000		1
Recorded investment of TDRs that defaulted during the period		$ 128,000		$ 128,000