Securities - Amortized Cost and Fair Value of Securities with Gross Unrealized Gains and Losses (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 243,056	$ 247,819
Gross Unrealized Gains	2,167	2,619
Gross Unrealized Losses	(437)	(1,946)
Fair Value	244,786	248,492
U.S. Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	38,861	25,833
Gross Unrealized Gains	124
Gross Unrealized Losses	(81)	(440)
Fair Value	38,904	25,393
U.S. Government Agency [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	100,798	120,154
Gross Unrealized Gains	215	391
Gross Unrealized Losses	(209)	(1,311)
Fair Value	100,804	119,234
Mortgage-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	27,593	29,067
Gross Unrealized Gains	472	557
Gross Unrealized Losses	(37)	(62)
Fair Value	28,028	29,562
State and Local Governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	75,804	72,765
Gross Unrealized Gains	1,356	1,671
Gross Unrealized Losses	(110)	(133)
Fair Value	$ 77,050	$ 74,303