INVESTMENTS (Schedule of Unrealized Gross Losses and Estimated Fair values of Investment Securities) (Details) $ in Thousands	Mar. 31, 2018 USD ($) security	Dec. 31, 2017 USD ($) security
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	328	209
Fair Value - Less Than 12 Months	$ 362,001	$ 324,078
Unrealized Losses - Less Than 12 Months	(7,992)	(3,089)
Fair Value - 12 Months of More	388,579	398,360
Unrealized Losses - 12 Months or More	(18,446)	(11,172)
Fair Value	750,580	722,438
Unrealized losses	(26,438)	(14,261)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	62,815	9,317
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(958)	(57)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	10,225	9,436
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(490)	(180)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	73,040	18,753
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(1,448)	(237)
AFS and HTM securities in Continuous Unrealized Loss Position	$ 823,600	$ 741,200
Unrealized Loss as a Percent of Fair Value	3.00%	2.00%
AFS and HTM Securities Unrealized Loss Accumulated in Investments	$ 27,900	$ 14,500
Mortgage Backed Securities and Collateralized Mortgage Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
AFS and HTM Securities Unrealized Loss Accumulated in Investments	18,400	11,200
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	236,851	221,466
Unrealized Losses - Less Than 12 Months	(5,661)	(2,393)
Fair Value - 12 Months of More	233,957	233,971
Unrealized Losses - 12 Months or More	(9,656)	(5,078)
Fair Value	470,808	455,437
Unrealized losses	(15,317)	(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	122,669	102,612
Unrealized Losses - Less Than 12 Months	(2,308)	(696)
Fair Value - 12 Months of More	154,622	164,389
Unrealized Losses - 12 Months or More	(8,790)	(6,094)
Fair Value	277,291	267,001
Unrealized losses	(11,098)	(6,790)
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value - Less Than 12 Months	965
Unrealized Losses - Less Than 12 Months	(19)
Fair Value - 12 Months of More	0
Unrealized Losses - 12 Months or More	0
Fair Value	965
Unrealized losses	(19)
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	62,815	9,317
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(958)	(57)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	10,225	9,436
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(490)	(180)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	73,040	18,753
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (1,448)	$ (237)