Allowance for Loan Losses - Troubled Debt Restructurings (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) SecurityLoan	Dec. 31, 2016 USD ($) SecurityLoan	Dec. 31, 2015 USD ($) SecurityLoan
Financing Receivable, Modifications [Line Items]
Number | SecurityLoan	26	24	23
Pre-mod outstanding principal balance	$ 7,111	$ 1,989	$ 3,979
Post-mod outstanding principal balance	6,978	2,035	3,973
Financial impact due to TDR taken as additional provision	$ 133	$ 326	$ 451
Number that defaulted during the period | SecurityLoan	5	3	10
Recorded investment of TDRs that subsequently defaulted during the 12 month period after modification	$ 624	$ 330	$ 736
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ (5)		$ (9)
Mortgage Loans on Real Estate [Member] | Residential [Member]
Financing Receivable, Modifications [Line Items]
Number | SecurityLoan	1	3	4
Pre-mod outstanding principal balance	$ 939	$ 650	$ 800
Post-mod outstanding principal balance	939	656	801
Financial impact due to TDR taken as additional provision	$ 169	$ 50	$ 8
Number that defaulted during the period | SecurityLoan	2	2	4
Recorded investment of TDRs that subsequently defaulted during the 12 month period after modification	$ 223	$ 101	$ 221
Mortgage Loans on Real Estate [Member] | Commercial [Member]
Financing Receivable, Modifications [Line Items]
Number | SecurityLoan	8	5	5
Pre-mod outstanding principal balance	$ 3,721	$ 423	$ 1,518
Post-mod outstanding principal balance	3,695	461	1,517
Financial impact due to TDR taken as additional provision	$ (111)	$ 46	$ (5)
Number that defaulted during the period | SecurityLoan	1		2
Recorded investment of TDRs that subsequently defaulted during the 12 month period after modification	$ 219		$ 280
Consumer [Member] | Home Equity Lines of Credit [Member]
Financing Receivable, Modifications [Line Items]
Number | SecurityLoan	3	9	2
Pre-mod outstanding principal balance	$ 187	$ 707	$ 301
Post-mod outstanding principal balance	187	709	$ 301
Financial impact due to TDR taken as additional provision	$ 27	$ 205
Number that defaulted during the period | SecurityLoan	1	1	3
Recorded investment of TDRs that subsequently defaulted during the 12 month period after modification	$ 127	$ 229	$ 182
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ (5)
Consumer [Member] | Home Equity Loans [Member]
Financing Receivable, Modifications [Line Items]
Number | SecurityLoan	1	1	2
Pre-mod outstanding principal balance	$ 252	$ 105	$ 315
Post-mod outstanding principal balance	$ 252	$ 105	321
Financial impact due to TDR taken as additional provision			$ 38
Number that defaulted during the period | SecurityLoan	1		1
Recorded investment of TDRs that subsequently defaulted during the 12 month period after modification	$ 55		$ 53
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions			$ (9)
Consumer [Member] | Other Consumer [Member]
Financing Receivable, Modifications [Line Items]
Number | SecurityLoan	1	2	2
Pre-mod outstanding principal balance	$ 14	$ 27	$ 89
Post-mod outstanding principal balance	14	27	89
Financial impact due to TDR taken as additional provision	$ 11	$ 2	$ 5
Consumer and Industrial [Member]
Financing Receivable, Modifications [Line Items]
Number | SecurityLoan	11	4	8
Pre-mod outstanding principal balance	$ 1,854	$ 77	$ 956
Post-mod outstanding principal balance	1,747	77	944
Financial impact due to TDR taken as additional provision	$ 37	$ 23	$ 405
Construction [Member] | Residential [Member]
Financing Receivable, Modifications [Line Items]
Number | SecurityLoan	1
Pre-mod outstanding principal balance	$ 144
Post-mod outstanding principal balance	$ 144