Securities Available-for-Sale - Summary of Securities Available-for-Sale (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 410,206	$ 448,210
Gross Unrealized Gains	1,255	1,630
Gross Unrealized Losses	1,247	1,450
Total, Fair Value	410,214	448,390
U.S. Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,999	1,999
Gross Unrealized Gains		1
Gross Unrealized Losses	1
Total, Fair Value	1,998	2,000
Small Business Administration [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,384	6,684
Gross Unrealized Gains	30	33
Gross Unrealized Losses	1
Total, Fair Value	6,413	6,717
U.S. Government Agency and Sponsored Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	248,557	336,158
Gross Unrealized Gains	1,031	1,387
Gross Unrealized Losses	239	452
Total, Fair Value	249,349	337,093
Privately Issued Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,538	1,894
Gross Unrealized Gains	4	5
Gross Unrealized Losses	14	25
Total, Fair Value	1,528	1,874
Obligations Issued by States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	147,975	97,657
Gross Unrealized Losses	876	873
Total, Fair Value	147,099	96,784
Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,600	3,600
Gross Unrealized Gains	29	24
Gross Unrealized Losses	116	100
Total, Fair Value	3,513	3,524
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	153	218
Gross Unrealized Gains	161	180
Total, Fair Value	$ 314	$ 398