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, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 745,748	$ 724,072
Gross Unrealized Gain	20,795	2,619
Gross Unrealized Loss	84	5,662
Fair Value	766,459	721,029
Held-to-Maturity Securities, Amortized Cost	166,398	165,862
Held-to-Maturity Securities, Fair Value	173,032	167,743
CMO [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	259,834	253,189
Gross Unrealized Gain	6,520	713
Gross Unrealized Loss	32	2,414
Fair Value	266,322	251,488
FNMA MBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	336,103	320,105
Gross Unrealized Gain	11,144	1,081
Gross Unrealized Loss	23	2,715
Fair Value	347,224	318,471
FHLMC MBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	84,550	99,350
Gross Unrealized Gain	2,343	405
Gross Unrealized Loss		313
Fair Value	86,893	99,442
GNMA MBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	26,181	20,387
Gross Unrealized Gain	641	420
Gross Unrealized Loss	29	93
Fair Value	26,793	20,714
GSE [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	39,080	31,041
Gross Unrealized Gain	147
Gross Unrealized Loss		127
Fair Value	39,227	30,914
State and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Held-to-Maturity Securities, Amortized Cost	166,398	165,862
Held-to-Maturity Securities, Gross Unrealized Gain	6,654	1,943
Held-to-Maturity Securities, Gross Unrealized Loss	20	62
Held-to-Maturity Securities, Fair Value	$ 173,032	$ 167,743