Allowance for Loan Losses - Impaired Financing Receivables - Recorded Investment and Unpaid Principal Balances (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Financing Receivable, Impaired [Line Items]
Impaired financing receivables, with no related allowance recorded, Recorded Investment	$ 6,031	$ 6,328
Impaired financing receivables, with no related allowance recorded, Unpaid Principal Balance	7,222	8,463
Impaired financing receivables, with no related allowance recorded, Related Allowance	0	0
Impaired financing receivables, with an allowance recorded, Recorded Investment	1,426	4,821
Impaired financing receivables, with an allowance recorded, Unpaid Principal Balance	1,443	5,209
Impaired financing receivables, with an allowance recorded, Related Allowance	315	1,023
Impaired financing receivables, Recorded Investment, Total	7,457	11,149
Impaired financing receivables, Unpaid Principal Balance, Total	8,665	13,672
Impaired financing receivables, Related Allowance, Total	315	1,023
Commercial and Agriculture [Member]
Financing Receivable, Impaired [Line Items]
Impaired financing receivables, with no related allowance recorded, Recorded Investment	871	1,377
Impaired financing receivables, with no related allowance recorded, Unpaid Principal Balance	1,036	1,504
Impaired financing receivables, with no related allowance recorded, Related Allowance	0	0
Impaired financing receivables, with an allowance recorded, Recorded Investment	23	927
Impaired financing receivables, with an allowance recorded, Unpaid Principal Balance	23	1,056
Impaired financing receivables, with an allowance recorded, Related Allowance	16	641
Impaired financing receivables, Recorded Investment, Total	894	2,304
Impaired financing receivables, Unpaid Principal Balance, Total	1,059	2,560
Impaired financing receivables, Related Allowance, Total	16	641
Commercial Real Estate Owner Occupied [Member]
Financing Receivable, Impaired [Line Items]
Impaired financing receivables, with no related allowance recorded, Recorded Investment	2,368	2,961
Impaired financing receivables, with no related allowance recorded, Unpaid Principal Balance	2,583	3,327
Impaired financing receivables, with no related allowance recorded, Related Allowance	0	0
Impaired financing receivables, with an allowance recorded, Recorded Investment	258	596
Impaired financing receivables, with an allowance recorded, Unpaid Principal Balance	258	643
Impaired financing receivables, with an allowance recorded, Related Allowance	4	57
Impaired financing receivables, Recorded Investment, Total	2,626	3,557
Impaired financing receivables, Unpaid Principal Balance, Total	2,841	3,970
Impaired financing receivables, Related Allowance, Total	4	57
Commercial Real Estate Non Owner Occupied [Member]
Financing Receivable, Impaired [Line Items]
Impaired financing receivables, with no related allowance recorded, Recorded Investment	1,828	92
Impaired financing receivables, with no related allowance recorded, Unpaid Principal Balance	2,094	140
Impaired financing receivables, with no related allowance recorded, Related Allowance	0	0
Impaired financing receivables, with an allowance recorded, Recorded Investment		2,083
Impaired financing receivables, with an allowance recorded, Unpaid Principal Balance		2,287
Impaired financing receivables, with an allowance recorded, Related Allowance		20
Impaired financing receivables, Recorded Investment, Total	1,828	2,175
Impaired financing receivables, Unpaid Principal Balance, Total	2,094	2,427
Impaired financing receivables, Related Allowance, Total		20
Residential Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired financing receivables, with no related allowance recorded, Recorded Investment	960	1,893
Impaired financing receivables, with no related allowance recorded, Unpaid Principal Balance	1,505	3,487
Impaired financing receivables, with no related allowance recorded, Related Allowance	0	0
Impaired financing receivables, with an allowance recorded, Recorded Investment	1,145	1,215
Impaired financing receivables, with an allowance recorded, Unpaid Principal Balance	1,162	1,223
Impaired financing receivables, with an allowance recorded, Related Allowance	295	305
Impaired financing receivables, Recorded Investment, Total	2,105	3,108
Impaired financing receivables, Unpaid Principal Balance, Total	2,667	4,710
Impaired financing receivables, Related Allowance, Total	295	305
Consumer and Other [Member]
Financing Receivable, Impaired [Line Items]
Impaired financing receivables, with no related allowance recorded, Recorded Investment	4	5
Impaired financing receivables, with no related allowance recorded, Unpaid Principal Balance	4	5
Impaired financing receivables, with no related allowance recorded, Related Allowance	0	0
Impaired financing receivables, Recorded Investment, Total	4	5
Impaired financing receivables, Unpaid Principal Balance, Total	$ 4	$ 5