INVESTMENTS (Schedule of Unrealized Gross Losses and Estimated Fair values of Investment Securities) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Fair Value - Less Than 12 Months	$ 371,387	$ 324,078
Unrealized Losses - Less Than 12 Months	(9,117)	(3,089)
Fair Value - 12 Months of More	390,045	398,360
Unrealized Losses - 12 Months or More	(21,003)	(11,172)
Fair Value	761,432	722,438
Unrealized losses	(30,120)	(14,261)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	63,572	9,317
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1,119)	(57)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	10,229	9,436
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(475)	(180)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	73,801	18,753
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(1,594)	(237)
Obligations of states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Fair Value - Less Than 12 Months	1,517
Unrealized Losses - Less Than 12 Months	(3)
Fair Value - 12 Months of More	0
Unrealized Losses - 12 Months or More	0
Fair Value	1,517
Unrealized losses	(3)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	63,572	9,317
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1,119)	(57)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	10,229	9,436
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(475)	(180)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	73,801	18,753
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(1,594)	(237)
Mortgage-backed securities issued or guaranteed by U.S. government-sponsored enterprises
Schedule of Available-for-sale Securities [Line Items]
Fair Value - Less Than 12 Months	240,208	221,466
Unrealized Losses - Less Than 12 Months	(6,256)	(2,393)
Fair Value - 12 Months of More	240,607	233,971
Unrealized Losses - 12 Months or More	(11,457)	(5,078)
Fair Value	480,815	455,437
Unrealized losses	(17,713)	(7,471)
Collateralized mortgage obligations issued or guaranteed by U.S. government-sponsored enterprises
Schedule of Available-for-sale Securities [Line Items]
Fair Value - Less Than 12 Months	128,698	102,612
Unrealized Losses - Less Than 12 Months	(2,837)	(696)
Fair Value - 12 Months of More	149,438	164,389
Unrealized Losses - 12 Months or More	(9,546)	(6,094)
Fair Value	278,136	267,001
Unrealized losses	(12,383)	$ (6,790)
Subordinated corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Fair Value - Less Than 12 Months	964
Unrealized Losses - Less Than 12 Months	(21)
Fair Value - 12 Months of More	0
Unrealized Losses - 12 Months or More	0
Fair Value	964
Unrealized losses	$ (21)

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

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