Investment Securities - Schedule of Amortized Cost and Estimated Fair Value of Available-for-Sale and Held-to-Maturity Investment Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 749,403	$ 739,404
Gross Unrealized Gain	8,545	4,314
Gross Unrealized Loss	1,217	3,594
Fair Value	756,731	740,124
Held-to-maturity Securities, Amortized Cost	138,060	126,168
Held-to-maturity Securities, Fair Value	138,138	126,171
Collateralized Mortgage Obligation (CMO) [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	251,729	193,672
Gross Unrealized Gain	2,538	874
Gross Unrealized Loss	600	1,614
Fair Value	253,667	192,932
Federal National Mortgage Association (FNMA) [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	296,373	291,606
Gross Unrealized Gain	3,838	2,053
Gross Unrealized Loss	439	1,106
Fair Value	299,772	292,553
Federal Home Loan Mortgage Corporation (FHLMC) [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	109,327	146,742
Gross Unrealized Gain	1,271	672
Gross Unrealized Loss	124	532
Fair Value	110,474	146,882
Government National Mortgage Association (GNMA) [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	60,930	77,364
Gross Unrealized Gain	834	701
Gross Unrealized Loss	53	268
Fair Value	61,711	77,797
U.S. Government and Government Sponsored Enterprises (GSE) [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	31,044	30,020
Gross Unrealized Gain	64	14
Gross Unrealized Loss	1	74
Fair Value	31,107	29,960
State and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Held-to-maturity Securities, Amortized Cost	138,060	126,168
Held-to-maturity Securities, Gross Unrealized Gain	670	3
Held-to-maturity Securities, Gross Unrealized Loss	592
Held-to-maturity Securities, Fair Value	$ 138,138	$ 126,171