Investment Securities - Schedule of Amortized Cost and Estimated Fair Value of Available-for-Sale and Held-to-Maturity Investment Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 724,072	$ 739,404
Gross Unrealized Gains	2,619	4,314
Gross Unrealized Losses	5,662	3,594
Fair Value	721,029	740,124
Held-to-Maturity Securities, Amortized Cost	165,862	126,168
Held-to-Maturity Securities, Fair Value	167,743	126,171
CMO [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	253,189	193,672
Gross Unrealized Gains	713	874
Gross Unrealized Losses	2,414	1,614
Fair Value	251,488	192,932
FNMA MBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	320,105	291,606
Gross Unrealized Gains	1,081	2,053
Gross Unrealized Losses	2,715	1,106
Fair Value	318,471	292,553
FHLMC MBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	99,350	146,742
Gross Unrealized Gains	405	672
Gross Unrealized Losses	313	532
Fair Value	99,442	146,882
GNMA MBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	20,387	77,364
Gross Unrealized Gains	420	701
Gross Unrealized Losses	93	268
Fair Value	20,714	77,797
State and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Held-to-Maturity Securities, Amortized Cost	165,862	126,168
Held-to-Maturity Securities, Gross Unrealized Gains	1,943	3
Held-to-Maturity Securities, Gross Unrealized Losses	62
Held-to-Maturity Securities, Fair Value	167,743	126,171
GSE [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	31,041	30,020
Gross Unrealized Gains		14
Gross Unrealized Losses	127	74
Fair Value	$ 30,914	$ 29,960

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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Amount of accumulated unrealized gain on investments in debt securities classified as held-to-maturity.

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Amount of accumulated unrealized loss on investments in debt securities classified as held-to-maturity.

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Fair value of investments in debt securities classified as held-to-maturity.

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