SECURITIES (Amortized Cost, Gross Unrealized Gains and Losses, and Estimated Fair Values of Investment Securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,267,205	$ 971,339
Gross Unrealized Gains	5,984	10,569
Gross Unrealized (Losses)	(20,010)	(7,686)
Estimated Fair Value	1,253,179	974,222
Obligations of States and Political Subdivisions
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	365,816	295,546
Gross Unrealized Gains	3,002	6,842
Gross Unrealized (Losses)	(4,179)	(564)
Estimated Fair Value	364,639	301,824
Corporate and Other Bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	122,903	113,625
Gross Unrealized Gains	1,175	1,131
Gross Unrealized (Losses)	(938)	(876)
Estimated Fair Value	123,140	113,880
Mortgage-Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	767,366	552,431
Gross Unrealized Gains	1,807	2,596
Gross Unrealized (Losses)	(14,761)	(6,169)
Estimated Fair Value	754,412	548,858
Other Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	11,120	9,737
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	(132)	(77)
Estimated Fair Value	$ 10,988	$ 9,660

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
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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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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
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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.

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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
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