Allowance for Loan Losses - Troubled Debt Restructurings (Detail) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2019 USD ($) SecurityLoan	Jun. 30, 2018 USD ($) SecurityLoan Branches	Jun. 30, 2019 USD ($) SecurityLoan Branches	Jun. 30, 2018 USD ($) Branches SecurityLoan
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan	6	3	10	6
Pre-mod outstanding principal balance	$ 1,847	$ 450	$ 2,145	$ 1,087
Post-mod outstanding principal balance	1,817	455	2,114	1,097
Financial impact due to TDR taken as additional provision	$ 29	$ 32	$ 31	$ 44
Number that defaulted during the period \| Branches		4	1	5
Recorded investment of TDRs that defaulted during the period		$ 340	$ 7	$ 509
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions		$ (2)		$ (2)
Mortgage Loans on Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan		1	1	2
Pre-mod outstanding principal balance		$ 34	$ 163	$ 417
Post-mod outstanding principal balance		34	$ 162	417
Financial impact due to TDR taken as additional provision		$ 34		$ 46
Number that defaulted during the period \| Branches				1
Recorded investment of TDRs that defaulted during the period				$ 169
Consumer [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan	2		3	2
Pre-mod outstanding principal balance	$ 93		$ 214	$ 254
Post-mod outstanding principal balance	95		215	$ 259
Financial impact due to TDR taken as additional provision	$ 27		$ 29
Commercial Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan	4	2	6	2
Pre-mod outstanding principal balance	$ 1,754	$ 416	$ 1,768	$ 416
Post-mod outstanding principal balance	1,722	421	1,737	421
Financial impact due to TDR taken as additional provision	$ 2	$ (2)	$ 2	$ (2)
Number that defaulted during the period \| Branches		4	1	4
Recorded investment of TDRs that defaulted during the period		$ 340	$ 7	$ 340
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions		$ (2)		$ (2)
Residential 1-4 Family [Member] \| Mortgage Loans on Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan			1
Pre-mod outstanding principal balance			$ 163
Post-mod outstanding principal balance			$ 162
Home Equity Loans [Member] \| Consumer [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan	1		2	1
Pre-mod outstanding principal balance	$ 28		$ 149	$ 121
Post-mod outstanding principal balance	27		147	$ 121
Financial impact due to TDR taken as additional provision	$ 27		$ 29
Commercial [Member] \| Mortgage Loans on Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan		1		2
Pre-mod outstanding principal balance		$ 34		$ 417
Post-mod outstanding principal balance		34		417
Financial impact due to TDR taken as additional provision		$ 34		$ 46
Number that defaulted during the period \| Branches				1
Recorded investment of TDRs that defaulted during the period				$ 169
Home Equity Lines of Credit [Member] \| Consumer [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan	1		1	1
Pre-mod outstanding principal balance	$ 65		$ 65	$ 133
Post-mod outstanding principal balance	$ 68		$ 68	$ 138

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Financial Impact due to troubled debt restructure charge offs or additional provision.

+ References

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

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953676-111524

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- Definition

Number of financing receivable contracts modified as troubled debt restructuring within previous 12 months, with subsequent payment default.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953682-111524

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Amount of financing receivable modified as troubled debt restructuring within previous 12 months, with subsequent payment default.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953682-111524

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