INVESTMENTS (Amortized Cost and Estimated Fair Values of Investments Available-for-sale) (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 728,365	$ 730,845
Gross Unrealized Gains	8,301	10,097
Gross Unrealized Losses	(7,159)	(7,388)
Estimated Fair Value	729,507	733,554
Marketable Equity Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	212	1,223
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	212	1,223
Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	728,153	729,622
Gross Unrealized Gains	8,301	10,097
Gross Unrealized Losses	(7,159)	(7,388)
Estimated Fair Value	729,295	732,331
Debt Securities \| U.S. Government Agencies
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	109,349	124,314
Gross Unrealized Gains	0	32
Gross Unrealized Losses	(2,781)	(2,556)
Estimated Fair Value	106,568	121,790
Debt Securities \| State and municipal
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	306,109	281,090
Gross Unrealized Gains	6,313	7,180
Gross Unrealized Losses	(169)	(586)
Estimated Fair Value	312,253	287,684
Debt Securities \| Mortgage-Backed
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	302,664	314,029
Gross Unrealized Gains	1,585	2,851
Gross Unrealized Losses	(4,209)	(4,169)
Estimated Fair Value	300,040	312,711
Debt Securities \| Trust Preferred
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	931	1,089
Gross Unrealized Gains	71	0
Gross Unrealized Losses	0	(77)
Estimated Fair Value	1,002	1,012
Available-for-Sale Securities \| Debt Securities \| Corporate Debt
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	9,100	9,100
Gross Unrealized Gains	332	34
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 9,432	$ 9,134

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
-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
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-Section 50
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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