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, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 825,488	$ 807,761
Gross Unrealized Gain	2,749	1,963
Gross Unrealized Loss	9,742	15,181
Investment securities available for sale	818,495	794,543
Held-to-Maturity Securities, Amortized Cost	162,598	164,346
Held-to-Maturity Securities, Fair Value	163,903	163,232
CMO
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	316,395	264,607
Gross Unrealized Gain	1,152	566
Gross Unrealized Loss	2,708	3,957
Investment securities available for sale	314,839	261,216
FNMA MBS
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	400,598	414,218
Gross Unrealized Gain	1,135	950
Gross Unrealized Loss	5,773	9,404
Investment securities available for sale	395,960	405,764
FHLMC MBS
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	77,815	64,709
Gross Unrealized Gain	168	135
Gross Unrealized Loss	839	1,330
Investment securities available for sale	77,144	63,514
GNMA MBS
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	27,050	28,540
Gross Unrealized Gain	294	303
Gross Unrealized Loss	405	427
Investment securities available for sale	26,939	28,416
Other investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	637	626
Gross Unrealized Gain	0	0
Gross Unrealized Loss	17	3
Investment securities available for sale	620	623
GSE
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,993	35,061
Gross Unrealized Gain	0	9
Gross Unrealized Loss	0	60
Investment securities available for sale	2,993	35,010
State and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Held-to-Maturity Securities, Amortized Cost	162,598	164,346
Held-to-Maturity Securities, Gross Unrealized Gain	1,525	271
Held-to-Maturity Securities, Gross Unrealized Loss	220	1,385
Held-to-Maturity Securities, Fair Value	$ 163,903	$ 163,232

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
-SubTopic 10
-Section 25
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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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Fair value of investments in debt securities classified as held-to-maturity.

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