Allowance for Loan Losses - TDRs (Detail) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018 USD ($) SecurityLoan	Sep. 30, 2017 USD ($) SecurityLoan	Sep. 30, 2018 USD ($) SecurityLoan	Sep. 30, 2017 USD ($) SecurityLoan
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan	7	15	13	25
Pre-mod outstanding principal balance	$ 2,007	$ 5,330	$ 3,094	$ 6,899
Post-mod outstanding principal balance	2,005	5,330	3,101	6,766
Financial impact due to TDR taken as additional provision	$ (308)	$ 211	$ (265)	$ 133
Number that defaulted during the period \| SecurityLoan	1	2	6	3
Recorded investment of TDRs that defaulted during the period	$ 128	$ 318	$ 637	$ 442
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions			$ (2)
Mortgage Loans on Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan	4	5	6	8
Pre-mod outstanding principal balance	$ 1,326	$ 3,825	$ 1,743	$ 4,448
Post-mod outstanding principal balance	1,324	3,825	1,741	4,421
Financial impact due to TDR taken as additional provision	$ (308)	$ 183	$ (262)	$ 58
Number that defaulted during the period \| SecurityLoan		2	1	3
Recorded investment of TDRs that defaulted during the period		$ 318	$ 169	$ 442
Consumer [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan	1	1	3	5
Pre-mod outstanding principal balance	$ 478	$ 252	$ 732	$ 453
Post-mod outstanding principal balance	$ 478	$ 252	$ 737	453
Financial impact due to TDR taken as additional provision				$ 38
Number that defaulted during the period \| SecurityLoan	1		1
Recorded investment of TDRs that defaulted during the period	$ 128		$ 128
Commercial Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan	2	8	4	11
Pre-mod outstanding principal balance	$ 203	$ 1,109	$ 619	$ 1,854
Post-mod outstanding principal balance	$ 203	1,109	623	1,748
Financial impact due to TDR taken as additional provision		$ 28	$ (3)	$ 37
Number that defaulted during the period \| SecurityLoan			4
Recorded investment of TDRs that defaulted during the period			$ 340
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions			$ (2)
Construction [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan		1		1
Pre-mod outstanding principal balance		$ 144		$ 144
Post-mod outstanding principal balance		$ 144		$ 144
Residential 1-4 Family [Member] \| Mortgage Loans on Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan		1		1
Pre-mod outstanding principal balance		$ 939		$ 939
Post-mod outstanding principal balance		939		939
Financial impact due to TDR taken as additional provision		$ 169		$ 169
Number that defaulted during the period \| SecurityLoan		1		2
Recorded investment of TDRs that defaulted during the period		$ 99		$ 223
Home Equity Loans [Member] \| Consumer [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan	1	1	2	1
Pre-mod outstanding principal balance	$ 478	$ 252	$ 599	$ 252
Post-mod outstanding principal balance	$ 478	$ 252	$ 599	$ 252
Other Consumer [Member] \| Consumer [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan				1
Pre-mod outstanding principal balance				$ 14
Post-mod outstanding principal balance				14
Financial impact due to TDR taken as additional provision				$ 11
Commercial [Member] \| Mortgage Loans on Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan	4	4	6	7
Pre-mod outstanding principal balance	$ 1,326	$ 2,886	$ 1,743	$ 3,509
Post-mod outstanding principal balance	1,324	2,886	1,741	3,482
Financial impact due to TDR taken as additional provision	$ (308)	$ 14	$ (262)	$ (111)
Number that defaulted during the period \| SecurityLoan		1	1	1
Recorded investment of TDRs that defaulted during the period		$ 219	$ 169	$ 219
Home Equity Lines of Credit [Member] \| Consumer [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan			1	3
Pre-mod outstanding principal balance			$ 133	$ 187
Post-mod outstanding principal balance			$ 138	187
Financial impact due to TDR taken as additional provision				$ 27
Number that defaulted during the period \| SecurityLoan	1		1
Recorded investment of TDRs that defaulted during the period	$ 128		$ 128
Residential [Member] \| Construction [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan		1		1
Pre-mod outstanding principal balance		$ 144		$ 144
Post-mod outstanding principal balance		$ 144		$ 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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