Allowance for Loan Losses - Troubled Debt Restructurings (Detail) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018 USD ($) SecurityLoan	Jun. 30, 2017 USD ($) SecurityLoan	Jun. 30, 2018 USD ($) SecurityLoan	Jun. 30, 2017 USD ($) SecurityLoan
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan	3	7	6	10
Pre-mod outstanding principal balance	$ 450	$ 1,435	$ 1,087	$ 1,569
Post-mod outstanding principal balance	456	1,302	1,099	1,436
Financial impact due to TDR taken as additional provision	$ 32	$ (182)	$ 44	$ (77)
Number that defaulted during the period \| SecurityLoan	4		5	1
Recorded investment of TDRs that defaulted during the period	$ 340		$ 509	$ 124
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	3	2	3
Pre-mod outstanding principal balance	$ 34	$ 623	$ 417	$ 623
Post-mod outstanding principal balance	34	596	418	596
Financial impact due to TDR taken as additional provision	$ 34	$ (125)	$ 46	$ (125)
Number that defaulted during the period \| SecurityLoan			1	1
Recorded investment of TDRs that defaulted during the period			$ 169	$ 124
Consumer [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan		2	2	4
Pre-mod outstanding principal balance		$ 167	$ 254	$ 201
Post-mod outstanding principal balance		167	$ 259	201
Financial impact due to TDR taken as additional provision		$ 27		$ 38
Commercial Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan	2	2	2	3
Pre-mod outstanding principal balance	$ 416	$ 645	$ 416	$ 745
Post-mod outstanding principal balance	422	539	422	639
Financial impact due to TDR taken as additional provision	$ (2)	$ (84)	$ (2)	$ 10
Number that defaulted during the period \| SecurityLoan	4		4
Recorded investment of TDRs that defaulted during the period	$ 340		$ 340
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ (2)		$ (2)
Home Equity Loans [Member] \| Consumer [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan			1
Pre-mod outstanding principal balance			$ 121
Post-mod outstanding principal balance			$ 121
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	1	3	2	3
Pre-mod outstanding principal balance	$ 34	$ 623	$ 417	$ 623
Post-mod outstanding principal balance	34	596	418	596
Financial impact due to TDR taken as additional provision	$ 34	$ (125)	$ 46	$ (125)
Number that defaulted during the period \| SecurityLoan			1	1
Recorded investment of TDRs that defaulted during the period			$ 169	$ 124
Home Equity Lines of Credit [Member] \| Consumer [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan		2	1	3
Pre-mod outstanding principal balance		$ 167	$ 133	$ 187
Post-mod outstanding principal balance		167	$ 138	187
Financial impact due to TDR taken as additional provision		$ 27		$ 27

Financial Impact due to troubled debt restructure additional provision.

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

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953676-111524

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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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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953682-111524

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Amount of investment related to financing receivables modified by troubled debt restructurings within the previous 12 months, for which there was a payment default.

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