Allowance for Loan Losses - Company's Recorded Investment and Customers' Unpaid Principal Balances for Impaired Loans, with Associated Allowance Amount (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment		$ 7,301		$ 7,301		$ 5,424
With no related allowance, Customers' Unpaid Principal Balance		7,647		7,647		5,504
With an allowance recorded, Recorded Investment		3,218		3,218		5,402
With an allowance recorded, Customers' Unpaid Principal Balance		3,276		3,276		5,689
With an allowance recorded, Related Allowance		843		843		1,534
Total Impaired Loans, Recorded Investment		10,519		10,519		10,826
Total Impaired Loans, Customers' Unpaid Principal Balance		10,923		10,923		11,193
Total Impaired Loans, Related Allowance		843		843		1,534
With no related allowance, Average Recorded Investment		7,454	$ 4,180	6,667	$ 3,616
With no related allowance, Interest Income Recognized		40	31	117	87
With an allowance recorded, Average Recorded Investment		3,227	3,490	3,216	3,458
With an allowance recorded, Interest Income Recognized		30	30	91	95
Total, Average Recorded Investment		10,681	7,670	9,883	7,074
Total, Interest Income Recognized		70	61	208	182
Construction, Land and Land Development
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment		1,532		1,532		445
With no related allowance, Customers' Unpaid Principal Balance		1,540		1,540		451
With an allowance recorded, Recorded Investment		248		248		262
With an allowance recorded, Customers' Unpaid Principal Balance		287		287		290
With an allowance recorded, Related Allowance		150		150		120
Total Impaired Loans, Recorded Investment		1,780		1,780		707
Total Impaired Loans, Customers' Unpaid Principal Balance		1,827		1,827		741
Total Impaired Loans, Related Allowance		150		150		120
With no related allowance, Average Recorded Investment		1,533	448	1,262	449
With no related allowance, Interest Income Recognized		14		41
With an allowance recorded, Average Recorded Investment		250	268	255	272
With an allowance recorded, Interest Income Recognized		1	1	3	4
Total, Average Recorded Investment		1,783	716	1,517	721
Total, Interest Income Recognized		15	1	44	4
Residential First Mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment		2,520		2,520		3,130
With no related allowance, Customers' Unpaid Principal Balance		2,552		2,552		3,166
With an allowance recorded, Recorded Investment		1,961		1,961		2,507
With an allowance recorded, Customers' Unpaid Principal Balance		1,961		1,961		2,507
With an allowance recorded, Related Allowance		366		366		308
Total Impaired Loans, Recorded Investment		4,481		4,481		5,637
Total Impaired Loans, Customers' Unpaid Principal Balance		4,513		4,513		5,673
Total Impaired Loans, Related Allowance		366		366		308
With no related allowance, Average Recorded Investment		2,618	2,061	2,432	1,813
With no related allowance, Interest Income Recognized		4	18	9	53
With an allowance recorded, Average Recorded Investment		1,965	1,741	1,956	1,748
With an allowance recorded, Interest Income Recognized		24	21	66	64
Total, Average Recorded Investment		4,583	3,802	4,388	3,561
Total, Interest Income Recognized		28	39	75	117
Residential Revolving and Junior Mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	[1]	951		951		233
With no related allowance, Customers' Unpaid Principal Balance	[1]	951		951		233
With an allowance recorded, Recorded Investment	[1]	232		232		258
With an allowance recorded, Customers' Unpaid Principal Balance	[1]	235		235		259
With an allowance recorded, Related Allowance	[1]	148		148		150
Total Impaired Loans, Recorded Investment	[1]	1,183		1,183		491
Total Impaired Loans, Customers' Unpaid Principal Balance	[1]	1,186		1,186		492
Total Impaired Loans, Related Allowance	[1]	148		148		150
With no related allowance, Average Recorded Investment	[1]	951	50	739	50
With no related allowance, Interest Income Recognized	[1]	9	1	30	2
With an allowance recorded, Average Recorded Investment	[1]	234	209	209	191
With an allowance recorded, Interest Income Recognized	[1]		2	4	6
Total, Average Recorded Investment	[1]	1,185	259	948	241
Total, Interest Income Recognized	[1]	9	3	34	8
Commercial Mortgages (Non-Owner Occupied)
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment		248		248		264
With no related allowance, Customers' Unpaid Principal Balance		248		248		264
Total Impaired Loans, Recorded Investment		248		248		264
Total Impaired Loans, Customers' Unpaid Principal Balance		248		248		264
With no related allowance, Average Recorded Investment		248	264	252	264
With no related allowance, Interest Income Recognized		4	4	11	12
Total, Average Recorded Investment		248	264	252	264
Total, Interest Income Recognized		4	4	11	12
Commercial Mortgages (Owner Occupied)
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment		2,050		2,050		1,352
With no related allowance, Customers' Unpaid Principal Balance		2,356		2,356		1,390
With an allowance recorded, Recorded Investment		684		684		2,091
With an allowance recorded, Customers' Unpaid Principal Balance		692		692		2,348
With an allowance recorded, Related Allowance		87		87		678
Total Impaired Loans, Recorded Investment		2,734		2,734		3,443
Total Impaired Loans, Customers' Unpaid Principal Balance		3,048		3,048		3,738
Total Impaired Loans, Related Allowance		87		87		678
With no related allowance, Average Recorded Investment		2,104	1,354	1,982	1,036
With no related allowance, Interest Income Recognized		9	8	26	20
With an allowance recorded, Average Recorded Investment		686	1,152	691	1,156
With an allowance recorded, Interest Income Recognized		5	6	17	21
Total, Average Recorded Investment		2,790	2,506	2,673	2,192
Total, Interest Income Recognized		14	14	43	41
Commercial and Industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
With an allowance recorded, Recorded Investment		93		93		284
With an allowance recorded, Customers' Unpaid Principal Balance		101		101		285
With an allowance recorded, Related Allowance		92		92		278
Total Impaired Loans, Recorded Investment		93		93		284
Total Impaired Loans, Customers' Unpaid Principal Balance		101		101		285
Total Impaired Loans, Related Allowance		92		92		$ 278
With an allowance recorded, Average Recorded Investment		92	120	105	91
With an allowance recorded, Interest Income Recognized				1
Total, Average Recorded Investment		$ 92	120	105	91
Total, Interest Income Recognized				$ 1
Consumer and Other Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Average Recorded Investment	[2]		3		4
Total, Average Recorded Investment	[2]		$ 3		$ 4

[1] Junior mortgages include equity lines.

[2] Includes credit cards.