Allowance for Loan Losses - Troubled Debt Restructurings (Detail) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2017 USD ($) SecurityLoan	Sep. 30, 2016 USD ($) SecurityLoan	Sep. 30, 2017 USD ($) SecurityLoan	Sep. 30, 2016 USD ($) SecurityLoan
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan	15	10	25	24
Pre-mod outstanding principal balance	$ 5,330	$ 674	$ 6,898	$ 1,989
Post-mod outstanding principal balance	5,330	682	6,765	2,035
Financial impact due to TDR taken as additional provision	$ 211	$ 21	$ 133	$ 326
Number that defaulted during the period \| SecurityLoan	2	2	3	3
Recorded investment of TDRs that defaulted during the period	$ 318	$ 243	$ 442	$ 330
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ 0	$ 0	$ 0	$ 0
Mortgage Loans on Real Estate [Member] \| Residential [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan	1	2	1	3
Pre-mod outstanding principal balance	$ 939	$ 318	$ 939	$ 650
Post-mod outstanding principal balance	939	324	939	656
Financial impact due to TDR taken as additional provision	$ 169	$ 6	$ 169	$ 50
Number that defaulted during the period \| SecurityLoan	1	1	2	2
Recorded investment of TDRs that defaulted during the period	$ 99	$ 14	$ 223	$ 101
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ 0	$ 0	$ 0	$ 0
Mortgage Loans on Real Estate [Member] \| Commercial [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan	4	2	7	3
Pre-mod outstanding principal balance	$ 2,886	$ 170	$ 3,509	$ 248
Post-mod outstanding principal balance	2,886	170	3,482	249
Financial impact due to TDR taken as additional provision	$ 14		$ (111)
Number that defaulted during the period \| SecurityLoan	1		1
Recorded investment of TDRs that defaulted during the period	$ 219		$ 219
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	0	$ 0	$ 0	$ 0
Consumer [Member] \| Home Equity Lines of Credit [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan		2	3	9
Pre-mod outstanding principal balance		$ 113	$ 187	$ 707
Post-mod outstanding principal balance		$ 114	187	709
Financial impact due to TDR taken as additional provision			27	$ 205
Number that defaulted during the period \| SecurityLoan		1		1
Recorded investment of TDRs that defaulted during the period		$ 229		$ 229
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ 0	0	$ 0	$ 0
Consumer [Member] \| Home Equity Loans [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan	1		1	3
Pre-mod outstanding principal balance	$ 252		$ 252	$ 280
Post-mod outstanding principal balance	252		252	317
Financial impact due to TDR taken as additional provision				46
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	0	$ 0	$ 0	$ 0
Consumer [Member] \| Other Consumer [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan		1	1	2
Pre-mod outstanding principal balance		$ 8	$ 14	$ 27
Post-mod outstanding principal balance		8	14	27
Financial impact due to TDR taken as additional provision			11	2
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ 0	0	$ 0	0
Consumer [Member] \| Auto Indirect [Member]
Financing Receivable, Modifications [Line Items]
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions		$ 0		$ 0
Consumer and Industrial [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan	8	3	11	4
Pre-mod outstanding principal balance	$ 1,109	$ 65	$ 1,854	$ 77
Post-mod outstanding principal balance	1,109	66	1,748	77
Financial impact due to TDR taken as additional provision	28	15	37	23
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ 0	0	$ 0	0
Construction [Member] \| Residential [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan	1		1
Pre-mod outstanding principal balance	$ 144		$ 144
Post-mod outstanding principal balance	144		144
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	0	0	0	0
Construction [Member] \| Commercial [Member]
Financing Receivable, Modifications [Line Items]
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ 0	$ 0	$ 0	$ 0

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+ References

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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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