Investment Securities - Analysis of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Investment Securities [Line Items]
Amortized Cost	$ 896,495	$ 924,054
Gross Unrealized Gains	8,906	2,998
Gross Unrealized Losses	(3,109)	(17,805)
Securities available for sale	902,292	909,247
US States and Political Subdivisions Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	17,738	27,592
Gross Unrealized Gains	171	126
Gross Unrealized Losses	0	(6)
Securities available for sale	17,909	27,712
Corporate Securities [Member]
Investment Securities [Line Items]
Amortized Cost	22,919	20,912
Gross Unrealized Gains	1,043	321
Gross Unrealized Losses	0	(221)
Securities available for sale	23,962	21,012
Obligations of U.S. Government-Sponsored Enterprises [Member] \| Mortgage - Backed Securities - Residential [Member]
Investment Securities [Line Items]
Amortized Cost	660,777	686,906
Gross Unrealized Gains	4,113	1,846
Gross Unrealized Losses	(2,943)	(15,391)
Securities available for sale	661,947	673,361
Obligations of U.S. Government-Sponsored Enterprises [Member] \| Other Government - Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	1,000	10,000
Gross Unrealized Gains	0	12
Gross Unrealized Losses	0	0
Securities available for sale	1,000	10,012
Obligations of U.S. Government [Member] \| Mortgage - Backed Securities - Commercial [Member]
Investment Securities [Line Items]
Amortized Cost	186,316	169,633
Gross Unrealized Gains	2,983	214
Gross Unrealized Losses	(166)	(2,103)
Securities available for sale	189,133	167,744
Obligations of U.S. Government [Member] \| Mortgage - Backed Securities - Residential [Member]
Investment Securities [Line Items]
Amortized Cost	7,745	9,011
Gross Unrealized Gains	596	479
Gross Unrealized Losses	0	(84)
Securities available for sale	$ 8,341	$ 9,406

Investment Securities [Line Items]

No definition available.

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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.

+ Details

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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

No definition available.

+ Details

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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)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

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- Definition

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=118939692&loc=d3e27161-111563

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