INVESTMENT SECURITIES - Schedule of Gross Unrealized Losses and Fair Value of Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Estimated Fair Value
Less Than 12 Months, Estimated Fair Value	$ 476,709	$ 299,794
12 Months or More, Estimated Fair Value	195,711	213,946
Total Estimated Fair Value	672,420	513,740
Unrealized Losses
Less Than 12 Months, Unrealized Losses	(8,896)	(2,551)
12 Months or More, Unrealized Losses	(7,505)	(4,341)
Total Unrealized Losses	(16,401)	(6,892)
State, county and municipal securities
Estimated Fair Value
Less Than 12 Months, Estimated Fair Value	54,182	33,976
12 Months or More, Estimated Fair Value	4,662	4,725
Total Estimated Fair Value	58,844	38,701
Unrealized Losses
Less Than 12 Months, Unrealized Losses	(448)	(115)
12 Months or More, Unrealized Losses	(86)	(48)
Total Unrealized Losses	(534)	(163)
Corporate debt securities
Estimated Fair Value
Less Than 12 Months, Estimated Fair Value	492	3,465
12 Months or More, Estimated Fair Value	18,501	18,853
Total Estimated Fair Value	18,993	22,318
Unrealized Losses
Less Than 12 Months, Unrealized Losses	(126)	(35)
12 Months or More, Unrealized Losses	(509)	(202)
Total Unrealized Losses	(635)	(237)
Mortgage-backed securities
Estimated Fair Value
Less Than 12 Months, Estimated Fair Value	422,035	262,353
12 Months or More, Estimated Fair Value	172,548	190,368
Total Estimated Fair Value	594,583	452,721
Unrealized Losses
Less Than 12 Months, Unrealized Losses	(8,322)	(2,401)
12 Months or More, Unrealized Losses	(6,910)	(4,091)
Total Unrealized Losses	$ (15,232)	$ (6,492)