Allowance for Loan Losses - Company's Recorded Investment and Customers' Unpaid Principal Balances for Impaired Loans, with Associated Allowance Amount (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment		$ 6,746	$ 5,424
With no related allowance, Customers' Unpaid Principal Balance		7,140	5,504
With an allowance recorded, Recorded Investment		3,669	5,402
With an allowance recorded, Customers' Unpaid Principal Balance		3,738	5,689
With an allowance recorded, Related Allowance		895	1,534
Total Impaired Loans, Recorded Investment		10,415	10,826
Total Impaired Loans, Customers' Unpaid Principal Balance		10,878	11,193
Total Impaired Loans, Related Allowance		895	1,534
With no related allowance, Average Recorded Investment		6,189	3,975
With no related allowance, Interest Income Recognized		149	123
With an allowance recorded, Average Recorded Investment		3,349	3,847
With an allowance recorded, Interest Income Recognized		127	147
Total, Average Recorded Investment		9,538	7,822
Total, Interest Income Recognized		276	270
Construction, Land and Land Development
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment		1,531	445
With no related allowance, Customers' Unpaid Principal Balance		1,539	451
With an allowance recorded, Recorded Investment		243	262
With an allowance recorded, Customers' Unpaid Principal Balance		286	290
With an allowance recorded, Related Allowance		145	120
Total Impaired Loans, Recorded Investment		1,774	707
Total Impaired Loans, Customers' Unpaid Principal Balance		1,825	741
Total Impaired Loans, Related Allowance		145	120
With no related allowance, Average Recorded Investment		1,316	448
With no related allowance, Interest Income Recognized		55
With an allowance recorded, Average Recorded Investment		253	270
With an allowance recorded, Interest Income Recognized		5	5
Total, Average Recorded Investment		1,569	718
Total, Interest Income Recognized		60	5
Residential First Mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment		2,112	3,130
With no related allowance, Customers' Unpaid Principal Balance		2,176	3,166
With an allowance recorded, Recorded Investment		1,951	2,507
With an allowance recorded, Customers' Unpaid Principal Balance		1,951	2,507
With an allowance recorded, Related Allowance		367	308
Total Impaired Loans, Recorded Investment		4,063	5,637
Total Impaired Loans, Customers' Unpaid Principal Balance		4,127	5,673
Total Impaired Loans, Related Allowance		367	308
With no related allowance, Average Recorded Investment		1,956	2,077
With no related allowance, Interest Income Recognized		14	74
With an allowance recorded, Average Recorded Investment		1,956	1,900
With an allowance recorded, Interest Income Recognized		90	90
Total, Average Recorded Investment		3,912	3,977
Total, Interest Income Recognized		104	164
Residential Revolving and Junior Mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	[1]	995	233
With no related allowance, Customers' Unpaid Principal Balance	[1]	999	233
With an allowance recorded, Recorded Investment	[1]	544	258
With an allowance recorded, Customers' Unpaid Principal Balance	[1]	546	259
With an allowance recorded, Related Allowance	[1]	199	150
Total Impaired Loans, Recorded Investment	[1]	1,539	491
Total Impaired Loans, Customers' Unpaid Principal Balance	[1]	1,545	492
Total Impaired Loans, Related Allowance	[1]	199	150
With no related allowance, Average Recorded Investment	[1]	808	87
With no related allowance, Interest Income Recognized	[1]	38	5
With an allowance recorded, Average Recorded Investment	[1]	218	204
With an allowance recorded, Interest Income Recognized	[1]	9	11
Total, Average Recorded Investment	[1]	1,026	291
Total, Interest Income Recognized	[1]	47	16
Commercial Mortgages (Non-Owner Occupied)
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment		248	264
With no related allowance, Customers' Unpaid Principal Balance		248	264
Total Impaired Loans, Recorded Investment		248	264
Total Impaired Loans, Customers' Unpaid Principal Balance		248	264
With no related allowance, Average Recorded Investment		251	264
With no related allowance, Interest Income Recognized		15	16
Total, Average Recorded Investment		251	264
Total, Interest Income Recognized		15	16
Commercial Mortgages (Owner Occupied)
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment		1,860	1,352
With no related allowance, Customers' Unpaid Principal Balance		2,178	1,390
With an allowance recorded, Recorded Investment		839	2,091
With an allowance recorded, Customers' Unpaid Principal Balance		854	2,348
With an allowance recorded, Related Allowance		92	678
Total Impaired Loans, Recorded Investment		2,699	3,443
Total Impaired Loans, Customers' Unpaid Principal Balance		3,032	3,738
Total Impaired Loans, Related Allowance		92	678
With no related allowance, Average Recorded Investment		1,858	1,099
With no related allowance, Interest Income Recognized		27	28
With an allowance recorded, Average Recorded Investment		819	1,343
With an allowance recorded, Interest Income Recognized		22	39
Total, Average Recorded Investment		2,677	2,442
Total, Interest Income Recognized		49	67
Commercial and Industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
With an allowance recorded, Recorded Investment		92	284
With an allowance recorded, Customers' Unpaid Principal Balance		101	285
With an allowance recorded, Related Allowance		92	278
Total Impaired Loans, Recorded Investment		92	284
Total Impaired Loans, Customers' Unpaid Principal Balance		101	285
Total Impaired Loans, Related Allowance		92	278
With an allowance recorded, Average Recorded Investment		103	130
With an allowance recorded, Interest Income Recognized		1	2
Total, Average Recorded Investment		103	130
Total, Interest Income Recognized		$ 1	$ 2

[1] Junior mortgages include equity lines.