Loans - Aging of Recorded Investment in Past Due Loans (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due	$ 7,476	$ 5,083
60-89 Days Past Due	1,876	2,144
Greater Than 90 Days Past Due	8,723	8,901
Total Past Due	18,075	16,128
Loans Not Past Due	1,341,900	1,343,468
Total	1,359,975	1,359,596
Commercial, Industrial, and Agricultural [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due	2,550	888
60-89 Days Past Due	69	588
Greater Than 90 Days Past Due	223	294
Total Past Due	2,842	1,770
Loans Not Past Due	433,463	426,688
Total	436,305	428,458
Commercial Mortgages [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due	2,395	20
60-89 Days Past Due	1,144	1,351
Greater Than 90 Days Past Due	4,447	4,323
Total Past Due	7,986	5,694
Loans Not Past Due	337,575	347,058
Total	345,561	352,752
Residential Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due	2,385	2,719
60-89 Days Past Due	614	1,191
Greater Than 90 Days Past Due	3,209	3,239
Total Past Due	6,208	7,149
Loans Not Past Due	498,361	495,168
Total	504,569	502,317
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due	146	265
60-89 Days Past Due	49	122
Greater Than 90 Days Past Due	844	1,045
Total Past Due	1,039	1,432
Loans Not Past Due	66,422	68,216
Total	67,461	69,648
Credit Cards [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due	0	0
60-89 Days Past Due	0	83
Greater Than 90 Days Past Due	0	0
Total Past Due	0	83
Loans Not Past Due	5,004	5,150
Total	5,004	5,233
Overdrafts [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due	0	0
60-89 Days Past Due	0	0
Greater Than 90 Days Past Due	0	0
Total Past Due	0	0
Loans Not Past Due	1,075	1,188
Total	$ 1,075	$ 1,188