INVESTMENTS (Amortized Cost and Estimated Fair Values of Investments Available-for-sale) (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 988,833	$ 728,365
Gross Unrealized Gains	5,091	8,301
Gross Unrealized Losses	(16,700)	(7,159)
Estimated Fair Value	977,224	729,507
Marketable Equity Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	568	212
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	568	212
Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	988,265	728,153
Gross Unrealized Gains	5,091	8,301
Gross Unrealized Losses	(16,700)	(7,159)
Estimated Fair Value	976,656	729,295
Debt Securities \| U.S. Government Agencies
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	213,734	109,349
Gross Unrealized Gains	1	0
Gross Unrealized Losses	(5,431)	(2,781)
Estimated Fair Value	208,304	106,568
Debt Securities \| State and municipal
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	328,324	306,109
Gross Unrealized Gains	3,918	6,313
Gross Unrealized Losses	(1,696)	(169)
Estimated Fair Value	330,546	312,253
Debt Securities \| Mortgage-Backed
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	436,797	302,664
Gross Unrealized Gains	920	1,585
Gross Unrealized Losses	(9,573)	(4,209)
Estimated Fair Value	428,144	300,040
Debt Securities \| Corporate Debt
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	9,100	9,100
Gross Unrealized Gains	252	332
Gross Unrealized Losses	0	0
Estimated Fair Value	9,352	9,432
Debt Securities \| Trust Preferred
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	310	931
Gross Unrealized Gains	0	71
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 310	$ 1,002

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
-SubTopic 10
-Section 50
-Paragraph 5
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-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
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-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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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
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section S99
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