INVESTMENTS (Schedule of Unrealized Gross Losses and Estimated Fair values of Investment Securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Fair Value - Less Than 12 Months	$ 250,616	$ 324,078
Unrealized Losses - Less Than 12 Months	(4,806)	(3,089)
Fair Value - 12 Months of More	509,704	398,360
Unrealized Losses - 12 Months or More	(29,056)	(11,172)
Fair Value	760,320	722,438
Unrealized losses	(33,862)	(14,261)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	67,328	9,317
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1,755)	(57)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	12,546	9,436
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(686)	(180)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	79,874	18,753
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(2,441)	(237)
Obligations of states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Fair Value - Less Than 12 Months	1,516
Unrealized Losses - Less Than 12 Months	(4)
Fair Value - 12 Months of More	0
Unrealized Losses - 12 Months or More	0
Fair Value	1,516
Unrealized losses	(4)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	67,328	9,317
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1,755)	(57)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	12,546	9,436
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(686)	(180)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	79,874	18,753
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(2,441)	(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	120,098	221,466
Unrealized Losses - Less Than 12 Months	(2,544)	(2,393)
Fair Value - 12 Months of More	328,180	233,971
Unrealized Losses - 12 Months or More	(17,344)	(5,078)
Fair Value	448,278	455,437
Unrealized losses	(19,888)	(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	123,951	102,612
Unrealized Losses - Less Than 12 Months	(2,220)	(696)
Fair Value - 12 Months of More	181,524	164,389
Unrealized Losses - 12 Months or More	(11,712)	(6,094)
Fair Value	305,475	267,001
Unrealized losses	(13,932)	$ (6,790)
Subordinated corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Fair Value - Less Than 12 Months	5,051
Unrealized Losses - Less Than 12 Months	(38)
Fair Value - 12 Months of More	0
Unrealized Losses - 12 Months or More	0
Fair Value	5,051
Unrealized losses	$ (38)

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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