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	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	$ 4,224		$ 3,912
With no related allowance, Customers' Unpaid Principal Balance	4,271		3,925
With no related allowance, Related Allowance	0		0
With an allowance recorded, Recorded Investment	2,634		2,744
With an allowance recorded, Customers' Unpaid Principal Balance	2,649		3,377
With an allowance recorded, Related Allowance	676		667
Total Impaired Loans, Recorded Investment	6,858		6,656
Total Impaired Loans, Customers' Unpaid Principal Balance	6,920		7,302
Total Impaired Loans, Related Allowance	676		667
With no related allowance, Average Recorded Investment	3,790		2,127
With no related allowance, Interest Income Recognized	147		113
With an allowance recorded, Average Recorded Investment	2,550		2,281
With an allowance recorded, Interest Income Recognized	115		123
Total, Average Recorded Investment	6,340		4,408
Total, Interest Income Recognized	262		236
Construction, Land and Land Development
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	450		453
With no related allowance, Customers' Unpaid Principal Balance	452		453
With no related allowance, Related Allowance	0		0
With an allowance recorded, Recorded Investment	277		151
With an allowance recorded, Customers' Unpaid Principal Balance	292		156
With an allowance recorded, Related Allowance	144		51
Total Impaired Loans, Recorded Investment	727		604
Total Impaired Loans, Customers' Unpaid Principal Balance	744		609
Total Impaired Loans, Related Allowance	144		51
With no related allowance, Average Recorded Investment	451		189
With no related allowance, Interest Income Recognized	3		1
With an allowance recorded, Average Recorded Investment	168		30
With an allowance recorded, Interest Income Recognized	4
Total, Average Recorded Investment	619		219
Total, Interest Income Recognized	7		1
Residential First Mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	1,568		1,053
With no related allowance, Customers' Unpaid Principal Balance	1,584		1,057
With no related allowance, Related Allowance	0		0
With an allowance recorded, Recorded Investment	2,173		2,198
With an allowance recorded, Customers' Unpaid Principal Balance	2,173		2,198
With an allowance recorded, Related Allowance	437		409
Total Impaired Loans, Recorded Investment	3,741		3,251
Total Impaired Loans, Customers' Unpaid Principal Balance	3,757		3,255
Total Impaired Loans, Related Allowance	437		409
With no related allowance, Average Recorded Investment	1,148		678
With no related allowance, Interest Income Recognized	49		48
With an allowance recorded, Average Recorded Investment	2,184		1,916
With an allowance recorded, Interest Income Recognized	100		108
Total, Average Recorded Investment	3,332		2,594
Total, Interest Income Recognized	149		156
Residential Revolving and Junior Mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	50	[1]
With no related allowance, Customers' Unpaid Principal Balance	50	[1]
With no related allowance, Related Allowance	0	[1]	0	[1]
With an allowance recorded, Recorded Investment	173	[1]	251	[1]
With an allowance recorded, Customers' Unpaid Principal Balance	173	[1]	879	[1]
With an allowance recorded, Related Allowance	84	[1]	173	[1]
Total Impaired Loans, Recorded Investment	223	[1]	251	[1]
Total Impaired Loans, Customers' Unpaid Principal Balance	223	[1]	879	[1]
Total Impaired Loans, Related Allowance	84	[1]	173	[1]
With no related allowance, Average Recorded Investment	10	[1]
With an allowance recorded, Average Recorded Investment	174	[1]	254	[1]
With an allowance recorded, Interest Income Recognized	9	[1]	8	[1]
Total, Average Recorded Investment	184	[1]	254	[1]
Total, Interest Income Recognized	9	[1]	8	[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		106
With no related allowance, Interest Income Recognized	16		8
Total, Average Recorded Investment	264		106
Total, Interest Income Recognized	16		8
Commercial Mortgages (Owner Occupied)
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	1,887		1,831
With no related allowance, Customers' Unpaid Principal Balance	1,916		1,840
With no related allowance, Related Allowance	0		0
With an allowance recorded, Recorded Investment			105
With an allowance recorded, Customers' Unpaid Principal Balance			105
With an allowance recorded, Related Allowance			1
Total Impaired Loans, Recorded Investment	1,887		1,936
Total Impaired Loans, Customers' Unpaid Principal Balance	1,916		1,945
Total Impaired Loans, Related Allowance			1
With no related allowance, Average Recorded Investment	1,911		1,092
With no related allowance, Interest Income Recognized	79		56
With an allowance recorded, Average Recorded Investment			21
With an allowance recorded, Interest Income Recognized			2
Total, Average Recorded Investment	1,911		1,113
Total, Interest Income Recognized	79		58
Commercial and Industrial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment			311
With no related allowance, Customers' Unpaid Principal Balance			311
With no related allowance, Related Allowance	0		0
Total Impaired Loans, Recorded Investment			311
Total Impaired Loans, Customers' Unpaid Principal Balance			311
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	5	[2]
With no related allowance, Customers' Unpaid Principal Balance	5	[2]
With no related allowance, Related Allowance	0	[2]	0	[2]
With an allowance recorded, Recorded Investment	11	[2]	39	[2]
With an allowance recorded, Customers' Unpaid Principal Balance	11	[2]	39	[2]
With an allowance recorded, Related Allowance	11	[2]	33	[2]
Total Impaired Loans, Recorded Investment	16	[2]	39	[2]
Total Impaired Loans, Customers' Unpaid Principal Balance	16	[2]	39	[2]
Total Impaired Loans, Related Allowance	11	[2]	33	[2]
With no related allowance, Average Recorded Investment	6	[2]
With an allowance recorded, Average Recorded Investment	24	[2]	60	[2]
With an allowance recorded, Interest Income Recognized	2	[2]	5	[2]
Total, Average Recorded Investment	30	[2]	60	[2]
Total, Interest Income Recognized	$ 2	[2]	$ 5	[2]

[1] Junior mortgages include equity lines.

[2] includes credit cards.