Investment Securities - Analysis of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015
Investment Securities [Line Items]
Amortized Cost	$ 790,082	$ 891,111
Gross Unrealized Gains	7,076	11,926
Gross Unrealized Losses	(18,546)	(16,477)
Estimated Fair Value of Available for Sale	778,612	886,560
US States and Political Subdivisions Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	27,075	27,066
Gross Unrealized Gains	195	532
Gross Unrealized Losses	(41)	0
Estimated Fair Value of Available for Sale	27,229	27,598
Corporate Securities [Member]
Investment Securities [Line Items]
Amortized Cost	5,903	1,897
Gross Unrealized Gains	416	422
Gross Unrealized Losses	0	0
Estimated Fair Value of Available for Sale	6,319	2,319
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Investment Securities [Line Items]
Amortized Cost	39,989	42,239
Gross Unrealized Gains	427	916
Gross Unrealized Losses	(7,124)	(7,497)
Estimated Fair Value of Available for Sale	33,292	35,658
Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	788,412	888,941
Gross Unrealized Gains	7,076	11,926
Gross Unrealized Losses	(18,546)	(16,477)
Estimated Fair Value of Available for Sale	776,942	884,390
Mortgage - Backed Securities - Residential [Member] \| US Government Agencies Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	15,143	20,034
Gross Unrealized Gains	1,481	2,071
Gross Unrealized Losses	(7)	(13)
Estimated Fair Value of Available for Sale	16,617	22,092
Mortgage - Backed Securities - Residential [Member] \| Obligations of U.S. Government-Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	683,601	778,476
Gross Unrealized Gains	4,557	7,983
Gross Unrealized Losses	(11,305)	(8,882)
Estimated Fair Value of Available for Sale	676,853	777,577
Mortgage - Backed Securities - Commercial [Member] \| Obligations of U.S. Government-Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	1	28
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value of Available for Sale	1	28
Other Government - Sponsored Enterprises [Member] \| Obligations of U.S. Government-Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	16,700	19,201
Gross Unrealized Gains	0	2
Gross Unrealized Losses	(69)	(85)
Estimated Fair Value of Available for Sale	16,631	19,118
Equity Securities [Member]
Investment Securities [Line Items]
Amortized Cost	1,670	2,170
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value of Available for Sale	$ 1,670	$ 2,170

Investment Securities [Line Items]

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26610-111562

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
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Amount of unrealized gain before deducting unrealized loss on investments in debt and equity securities classified as available-for-sale securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
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Amount of unrealized loss before deducting unrealized gain on investments in available-for-sale securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
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