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, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment		$ 4,280		$ 4,280		$ 4,224
With no related allowance, Customers' Unpaid Principal Balance		4,349		4,349		4,271
With no related allowance, Related Allowance		0		0		0
With an allowance recorded, Recorded Investment		3,520		3,520		2,634
With an allowance recorded, Customers' Unpaid Principal Balance		3,555		3,555		2,649
With an allowance recorded, Related Allowance		830		830		676
Total Impaired Loans, Recorded Investment		7,800		7,800		6,858
Total Impaired Loans, Customers' Unpaid Principal Balance		7,904		7,904		6,920
Total Impaired Loans, Related Allowance		830		830		676
With no related allowance, Average Recorded Investment		4,180	$ 3,664	3,616	$ 3,683
With no related allowance, Interest Income Recognized		31	34	87	106
With an allowance recorded, Average Recorded Investment		3,490	2,801	3,458	2,675
With an allowance recorded, Interest Income Recognized		30	30	95	85
Total, Average Recorded Investment		7,670	6,465	7,074	6,358
Total, Interest Income Recognized		61	64	182	191
Construction, Land and Land Development
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment		447		447		450
With no related allowance, Customers' Unpaid Principal Balance		451		451		452
With no related allowance, Related Allowance		0		0		0
With an allowance recorded, Recorded Investment		267		267		277
With an allowance recorded, Customers' Unpaid Principal Balance		292		292		292
With an allowance recorded, Related Allowance		126		126		144
Total Impaired Loans, Recorded Investment		714		714		727
Total Impaired Loans, Customers' Unpaid Principal Balance		743		743		744
Total Impaired Loans, Related Allowance		126		126		144
With no related allowance, Average Recorded Investment		448	451	449	452
With no related allowance, Interest Income Recognized			1		2
With an allowance recorded, Average Recorded Investment		268	426	272	288
With an allowance recorded, Interest Income Recognized		1	1	4	2
Total, Average Recorded Investment		716	877	721	740
Total, Interest Income Recognized		1	2	4	4
Residential First Mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment		2,164		2,164		1,568
With no related allowance, Customers' Unpaid Principal Balance		2,193		2,193		1,584
With no related allowance, Related Allowance		0		0		0
With an allowance recorded, Recorded Investment		1,739		1,739		2,173
With an allowance recorded, Customers' Unpaid Principal Balance		1,739		1,739		2,173
With an allowance recorded, Related Allowance		273		273		437
Total Impaired Loans, Recorded Investment		3,903		3,903		3,741
Total Impaired Loans, Customers' Unpaid Principal Balance		3,932		3,932		3,757
Total Impaired Loans, Related Allowance		273		273		437
With no related allowance, Average Recorded Investment		2,061	1,038	1,813	1,043
With no related allowance, Interest Income Recognized		18	10	53	31
With an allowance recorded, Average Recorded Investment		1,741	2,181	1,748	2,187
With an allowance recorded, Interest Income Recognized		21	26	64	74
Total, Average Recorded Investment		3,802	3,219	3,561	3,230
Total, Interest Income Recognized		39	36	117	105
Residential Revolving and Junior Mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	[1]	50		50		50
With no related allowance, Customers' Unpaid Principal Balance	[1]	50		50		50
With no related allowance, Related Allowance	[1]	0		0		0
With an allowance recorded, Recorded Investment	[1]	245		245		173
With an allowance recorded, Customers' Unpaid Principal Balance	[1]	245		245		173
With an allowance recorded, Related Allowance	[1]	141		141		84
Total Impaired Loans, Recorded Investment	[1]	295		295		223
Total Impaired Loans, Customers' Unpaid Principal Balance	[1]	295		295		223
Total Impaired Loans, Related Allowance	[1]	141		141		84
With no related allowance, Average Recorded Investment	[1]	50		50
With no related allowance, Interest Income Recognized	[1]	1		2
With an allowance recorded, Average Recorded Investment	[1]	209	173	191	174
With an allowance recorded, Interest Income Recognized	[1]	2	2	6	7
Total, Average Recorded Investment	[1]	259	173	241	174
Total, Interest Income Recognized	[1]	3	2	8	7
Commercial Mortgages (Non-Owner Occupied)
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment		264		264		264
With no related allowance, Customers' Unpaid Principal Balance		264		264		264
With no related allowance, Related Allowance		0		0		0
Total Impaired Loans, Recorded Investment		264		264		264
Total Impaired Loans, Customers' Unpaid Principal Balance		264		264		264
With no related allowance, Average Recorded Investment		264	264	264	264
With no related allowance, Interest Income Recognized		4	4	12	12
Total, Average Recorded Investment		264	264	264	264
Total, Interest Income Recognized		4	4	12	12
Commercial Mortgages (Owner Occupied)
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment		1,353		1,353		1,887
With no related allowance, Customers' Unpaid Principal Balance		1,389		1,389		1,916
With no related allowance, Related Allowance		0		0		0
With an allowance recorded, Recorded Investment		1,150		1,150
With an allowance recorded, Customers' Unpaid Principal Balance		1,158		1,158
With an allowance recorded, Related Allowance		171		171
Total Impaired Loans, Recorded Investment		2,503		2,503		1,887
Total Impaired Loans, Customers' Unpaid Principal Balance		2,547		2,547		1,916
Total Impaired Loans, Related Allowance		171		171
With no related allowance, Average Recorded Investment		1,354	1,905	1,036	1,917
With no related allowance, Interest Income Recognized		8	19	20	61
With an allowance recorded, Average Recorded Investment		1,152		1,156
With an allowance recorded, Interest Income Recognized		6		21
Total, Average Recorded Investment		2,506	1,905	2,192	1,917
Total, Interest Income Recognized		14	19	41	61
Commercial and Industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Related Allowance		0		0		0
With an allowance recorded, Recorded Investment		119		119
With an allowance recorded, Customers' Unpaid Principal Balance		121		121
With an allowance recorded, Related Allowance		119		119
Total Impaired Loans, Recorded Investment		119		119
Total Impaired Loans, Customers' Unpaid Principal Balance		121		121
Total Impaired Loans, Related Allowance		119		119
With an allowance recorded, Average Recorded Investment		120		91
Total, Average Recorded Investment		120		91
Consumer and Other Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	[2]	2		2		5
With no related allowance, Customers' Unpaid Principal Balance	[2]	2		2		5
With no related allowance, Related Allowance	[2]	0		0		0
With an allowance recorded, Recorded Investment	[2]					11
With an allowance recorded, Customers' Unpaid Principal Balance	[2]					11
With an allowance recorded, Related Allowance	[2]					11
Total Impaired Loans, Recorded Investment	[2]	2		2		16
Total Impaired Loans, Customers' Unpaid Principal Balance	[2]	2		2		16
Total Impaired Loans, Related Allowance	[2]					$ 11
With no related allowance, Average Recorded Investment	[2]	3	6	4	7
With an allowance recorded, Average Recorded Investment	[2]		21		26
With an allowance recorded, Interest Income Recognized	[2]		1		2
Total, Average Recorded Investment	[2]	$ 3	27	$ 4	33
Total, Interest Income Recognized	[2]		$ 1		$ 2

[1] Junior mortgages include equity lines.

[2] Includes credit cards.