Investment Securities - Analysis of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Investment Securities [Line Items]
Amortized Cost	$ 740,846	$ 790,082
Gross Unrealized Gains	4,854	7,076
Gross Unrealized Losses	(14,342)	(18,546)
Estimated Fair Value of Available for Sale	731,358	778,612
US States and Political Subdivisions Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	27,083	27,075
Gross Unrealized Gains	327	195
Gross Unrealized Losses	0	(41)
Estimated Fair Value of Available for Sale	27,410	27,229
Corporate Securities [Member]
Investment Securities [Line Items]
Amortized Cost	15,907	5,903
Gross Unrealized Gains	590	416
Gross Unrealized Losses	(4)	0
Estimated Fair Value of Available for Sale	16,493	6,319
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Investment Securities [Line Items]
Amortized Cost	27,499	39,989
Gross Unrealized Gains	526	427
Gross Unrealized Losses	(4,379)	(7,124)
Estimated Fair Value of Available for Sale	23,646	33,292
Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	739,176	788,412
Gross Unrealized Gains	4,854	7,076
Gross Unrealized Losses	(14,342)	(18,546)
Estimated Fair Value of Available for Sale	729,688	776,942
Mortgage - Backed Securities - Residential [Member] \| US Government Agencies Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	10,556	15,143
Gross Unrealized Gains	789	1,481
Gross Unrealized Losses	(7)	(7)
Estimated Fair Value of Available for Sale	11,338	16,617
Mortgage - Backed Securities - Residential [Member] \| Obligations of U.S. Government-Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	632,422	683,601
Gross Unrealized Gains	2,622	4,557
Gross Unrealized Losses	(9,489)	(11,305)
Estimated Fair Value of Available for Sale	625,555	676,853
Mortgage - Backed Securities - Commercial [Member] \| Obligations of U.S. Government-Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	0	1
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value of Available for Sale	0	1
Other Government - Sponsored Enterprises [Member] \| Obligations of U.S. Government-Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	1,098	16,700
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1)	(69)
Estimated Fair Value of Available for Sale	1,097	16,631
Equity Securities [Member]
Investment Securities [Line Items]
Amortized Cost	1,670	1,670
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value of Available for Sale	1,670	1,670
US Government Agencies Debt Securities [Member] \| Mortgage - Backed Securities - Commercial [Member]
Investment Securities [Line Items]
Amortized Cost	24,611	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(462)	0
Estimated Fair Value of Available for Sale	$ 24,149	$ 0

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=75030427&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=82897076&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=82896770&loc=d3e26610-111562

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ 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)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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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
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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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)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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