SECURITIES - Schedule of Gross Unrealized Losses and Fair Value of Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Estimated Fair Value
Less Than 12 Months	$ 299,794	$ 480,324
12 Months or More	213,946	12,284
Estimated Fair Value	513,740	492,608
Unrealized Losses
Less Than 12 Months	(2,551)	(7,009)
12 Months or More	(4,341)	(384)
Unrealized Losses	(6,892)	(7,393)
State, county and municipal securities
Estimated Fair Value
Less Than 12 Months	33,976	47,647
12 Months or More	4,725	0
Estimated Fair Value	38,701	47,647
Unrealized Losses
Less Than 12 Months	(115)	(469)
12 Months or More	(48)	0
Unrealized Losses	(163)	(469)
Corporate debt securities
Estimated Fair Value
Less Than 12 Months	3,465	18,377
12 Months or More	18,853	493
Estimated Fair Value	22,318	18,870
Unrealized Losses
Less Than 12 Months	(35)	(363)
12 Months or More	(202)	(7)
Unrealized Losses	(237)	(370)
Mortgage-backed securities
Estimated Fair Value
Less Than 12 Months	262,353	414,300
12 Months or More	190,368	11,791
Estimated Fair Value	452,721	426,091
Unrealized Losses
Less Than 12 Months	(2,401)	(6,177)
12 Months or More	(4,091)	(377)
Unrealized Losses	$ (6,492)	$ (6,554)