Investment Securities - Analysis of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Investment Securities [Line Items]
Amortized Cost	$ 924,054	$ 740,846
Gross Unrealized Gains	2,998	4,854
Gross Unrealized Losses	(17,805)	(14,342)
Securities available for sale	909,247	731,358
US States and Political Subdivisions Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	27,592	27,083
Gross Unrealized Gains	126	327
Gross Unrealized Losses	(6)	0
Securities available for sale	27,712	27,410
Corporate Securities [Member]
Investment Securities [Line Items]
Amortized Cost	20,912	15,907
Gross Unrealized Gains	321	590
Gross Unrealized Losses	(221)	(4)
Securities available for sale	21,012	16,493
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Investment Securities [Line Items]
Amortized Cost	0	27,499
Gross Unrealized Gains	0	526
Gross Unrealized Losses	0	(4,379)
Securities available for sale	0	23,646
Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	924,054	739,176
Gross Unrealized Gains	2,998	4,854
Gross Unrealized Losses	(17,805)	(14,342)
Securities available for sale	909,247	729,688
Equity Securities [Member]
Investment Securities [Line Items]
Amortized Cost	0	1,670
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Securities available for sale	0	1,670
Obligations of U.S. Government-Sponsored Enterprises [Member] \| Mortgage - Backed Securities - Commercial [Member]
Investment Securities [Line Items]
Amortized Cost	0	0
Obligations of U.S. Government-Sponsored Enterprises [Member] \| Mortgage - Backed Securities - Residential [Member]
Investment Securities [Line Items]
Amortized Cost	686,906	632,422
Gross Unrealized Gains	1,846	2,622
Gross Unrealized Losses	(15,391)	(9,489)
Securities available for sale	673,361	625,555
Obligations of U.S. Government-Sponsored Enterprises [Member] \| Other Government - Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	10,000	1,098
Gross Unrealized Gains	12	0
Gross Unrealized Losses	0	(1)
Securities available for sale	10,012	1,097
Obligations of U.S. Government [Member] \| Mortgage - Backed Securities - Commercial [Member]
Investment Securities [Line Items]
Amortized Cost	169,633	24,611
Gross Unrealized Gains	214	0
Gross Unrealized Losses	(2,103)	(462)
Securities available for sale	167,744	24,149
Obligations of U.S. Government [Member] \| Mortgage - Backed Securities - Residential [Member]
Investment Securities [Line Items]
Amortized Cost	9,011	10,556
Gross Unrealized Gains	479	789
Gross Unrealized Losses	(84)	(7)
Securities available for sale	$ 9,406	$ 11,338

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

No definition available.

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- Definition

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

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- Definition

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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
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Amount of unrealized loss before deducting unrealized gain on investments in available-for-sale securities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-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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