Investment Securities - Schedule of Amortized Cost and Estimated Fair Value of Available-for-Sale and Held-to-Maturity Investment Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 784,307	$ 739,404
Gross Unrealized Gain	2,758	4,314
Gross Unrealized Loss	5,319	3,594
Fair Value	781,746	740,124
Held-to-maturity Securities, Amortized Cost	120,697	126,168
Held-to-maturity Securities, Fair Value	118,804	126,171
Collateralized Mortgage Obligation (CMO) [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	254,596	193,672
Gross Unrealized Gain	773	874
Gross Unrealized Loss	2,033	1,614
Fair Value	253,336	192,932
Federal National Mortgage Association (FNMA) [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	302,595	291,606
Gross Unrealized Gain	1,002	2,053
Gross Unrealized Loss	2,472	1,106
Fair Value	301,125	292,553
Federal Home Loan Mortgage Corporation (FHLMC) [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	134,161	146,742
Gross Unrealized Gain	354	672
Gross Unrealized Loss	621	532
Fair Value	133,894	146,882
Government National Mortgage Association (GNMA) [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	63,922	77,364
Gross Unrealized Gain	594	701
Gross Unrealized Loss	178	268
Fair Value	64,338	77,797
U.S. Government and Government Sponsored Enterprises (GSE) [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	29,033	30,020
Gross Unrealized Gain	35	14
Gross Unrealized Loss	15	74
Fair Value	29,053	29,960
State and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Held-to-maturity Securities, Amortized Cost	120,697	126,168
Held-to-maturity Securities, Gross Unrealized Gain	152	3
Held-to-maturity Securities, Gross Unrealized Loss	2,045
Held-to-maturity Securities, Fair Value	$ 118,804	$ 126,171