Securities - Amortized Cost and Fair Value of Securities with Gross Unrealized Gains and Losses (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 253,158	$ 324,500
Gross Unrealized Gains	2,934	5,023
Gross Unrealized Losses	(2,973)	(5,014)
Fair Value	253,119	324,509
U.S. Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	25,892	26,067
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(610)	(795)
Fair Value	25,282	25,272
U.S. Government agency [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	120,245	174,772
Gross Unrealized Gains	534	1,386
Gross Unrealized Losses	(1,823)	(3,186)
Fair Value	118,956	172,972
Mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	30,773	44,638
Gross Unrealized Gains	536	728
Gross Unrealized Losses	(176)	(574)
Fair Value	31,133	44,792
State and local governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	76,248	79,023
Gross Unrealized Gains	1,864	2,909
Gross Unrealized Losses	(364)	(459)
Fair Value	$ 77,748	$ 81,473