Allowance for Loan Losses - Company's Recorded Investment and Customers' Unpaid Principal Balances for Impaired Loans, with Associated Allowance Amount (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014		Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	$ 3,879				$ 4,224
With no related allowance, Customers' Unpaid Principal Balance	3,933				4,271
With no related allowance, Related Allowance	0				0
With an allowance recorded, Recorded Investment	3,072				2,634
With an allowance recorded, Customers' Unpaid Principal Balance	3,093				2,649
With an allowance recorded, Related Allowance	794				676
Total Impaired Loans, Recorded Investment	6,951				6,858
Total Impaired Loans, Customers' Unpaid Principal Balance	7,026				6,920
Total Impaired Loans, Related Allowance	794				676
With no related allowance, Average Recorded Investment	3,827		4,394
With no related allowance, Interest Income Recognized	31		57
With an allowance recorded, Average Recorded Investment	3,077		2,731
With an allowance recorded, Interest Income Recognized	33		26
Total, Average Recorded Investment	6,904		7,125
Total, Interest Income Recognized	64		83
Construction, Land and Land Development
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	450				450
With no related allowance, Customers' Unpaid Principal Balance	453				452
With no related allowance, Related Allowance	0				0
With an allowance recorded, Recorded Investment	274				277
With an allowance recorded, Customers' Unpaid Principal Balance	292				292
With an allowance recorded, Related Allowance	141				144
Total Impaired Loans, Recorded Investment	724				727
Total Impaired Loans, Customers' Unpaid Principal Balance	745				744
Total Impaired Loans, Related Allowance	141				144
With no related allowance, Average Recorded Investment	450		452
With no related allowance, Interest Income Recognized			1
With an allowance recorded, Average Recorded Investment	275		150
With an allowance recorded, Interest Income Recognized	1
Total, Average Recorded Investment	725		602
Total, Interest Income Recognized	1		1
Residential First Mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	1,564				1,568
With no related allowance, Customers' Unpaid Principal Balance	1,582				1,584
With no related allowance, Related Allowance	0				0
With an allowance recorded, Recorded Investment	2,166				2,173
With an allowance recorded, Customers' Unpaid Principal Balance	2,167				2,173
With an allowance recorded, Related Allowance	429				437
Total Impaired Loans, Recorded Investment	3,730				3,741
Total Impaired Loans, Customers' Unpaid Principal Balance	3,749				3,757
Total Impaired Loans, Related Allowance	429				437
With no related allowance, Average Recorded Investment	1,566		1,485
With no related allowance, Interest Income Recognized	18		22
With an allowance recorded, Average Recorded Investment	2,169		2,193
With an allowance recorded, Interest Income Recognized	26		21
Total, Average Recorded Investment	3,735		3,678
Total, Interest Income Recognized	44		43
Residential Revolving and Junior Mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	50	[1]			50	[1]
With no related allowance, Customers' Unpaid Principal Balance	50	[1]			50	[1]
With no related allowance, Related Allowance	0	[1]			0	[1]
With an allowance recorded, Recorded Investment	173	[1]			173	[1]
With an allowance recorded, Customers' Unpaid Principal Balance	173	[1]			173	[1]
With an allowance recorded, Related Allowance	83	[1]			84	[1]
Total Impaired Loans, Recorded Investment	223	[1]			223	[1]
Total Impaired Loans, Customers' Unpaid Principal Balance	223	[1]			223	[1]
Total Impaired Loans, Related Allowance	83	[1]			84	[1]
With no related allowance, Average Recorded Investment	50	[1]	49	[1]
With no related allowance, Interest Income Recognized	1	[1]	1	[1]
With an allowance recorded, Average Recorded Investment	173	[1]	244	[1]
With an allowance recorded, Interest Income Recognized	2	[1]	2	[1]
Total, Average Recorded Investment	223	[1]	293	[1]
Total, Interest Income Recognized	3	[1]	3	[1]
Commercial Mortgages (Non-Owner Occupied)
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	264				264
With no related allowance, Customers' Unpaid Principal Balance	264				264
With no related allowance, Related Allowance	0				0
Total Impaired Loans, Recorded Investment	264				264
Total Impaired Loans, Customers' Unpaid Principal Balance	264				264
With no related allowance, Average Recorded Investment	264		264
With no related allowance, Interest Income Recognized	4		4
Total, Average Recorded Investment	264		264
Total, Interest Income Recognized	4		4
Commercial Mortgages (Owner Occupied)
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	1,423				1,887
With no related allowance, Customers' Unpaid Principal Balance	1,456				1,916
With no related allowance, Related Allowance	0				0
With an allowance recorded, Recorded Investment	448
With an allowance recorded, Customers' Unpaid Principal Balance	450
With an allowance recorded, Related Allowance	130
Total Impaired Loans, Recorded Investment	1,871				1,887
Total Impaired Loans, Customers' Unpaid Principal Balance	1,906				1,916
Total Impaired Loans, Related Allowance	130
With no related allowance, Average Recorded Investment	1,430		2,144
With no related allowance, Interest Income Recognized	8		29
With an allowance recorded, Average Recorded Investment	449		105
With an allowance recorded, Interest Income Recognized	4		2
Total, Average Recorded Investment	1,879		2,249
Total, Interest Income Recognized	12		31
Commercial and Industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	124
With no related allowance, Customers' Unpaid Principal Balance	124
With no related allowance, Related Allowance	0				0
Total Impaired Loans, Recorded Investment	124
Total Impaired Loans, Customers' Unpaid Principal Balance	124
With no related allowance, Average Recorded Investment	62
Total, Average Recorded Investment	62
Consumer Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	4	[2]			5	[2]
With no related allowance, Customers' Unpaid Principal Balance	4	[2]			5	[2]
With no related allowance, Related Allowance	0	[2]			0	[2]
With an allowance recorded, Recorded Investment	11	[2]			11	[2]
With an allowance recorded, Customers' Unpaid Principal Balance	11	[2]			11	[2]
With an allowance recorded, Related Allowance	11	[2]			11	[2]
Total Impaired Loans, Recorded Investment	15	[2]			16	[2]
Total Impaired Loans, Customers' Unpaid Principal Balance	15	[2]			16	[2]
Total Impaired Loans, Related Allowance	11	[2]			11	[2]
With no related allowance, Average Recorded Investment	5	[2]
With an allowance recorded, Average Recorded Investment	11	[2]	39	[2]
With an allowance recorded, Interest Income Recognized			1	[2]
Total, Average Recorded Investment	16	[2]	39	[2]
Total, Interest Income Recognized			$ 1	[2]

[1] Junior mortgages include equity lines.

[2] Includes credit cards.