INVESTMENT SECURITIES - Schedule of Gross Unrealized Losses and Fair Value of Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Estimated Fair Value
Less than 12 months, estimated fair value	$ 93,235	$ 160,820
12 months or more, estimated fair value	499,703	487,574
Total estimated fair value	592,938	648,394
Unrealized Losses
Less than 12 months, unrealized losses	(237)	(743)
12 months or more, unrealized losses	(9,071)	(12,067)
Total unrealized losses	(9,308)	(12,810)
State, county and municipal securities
Estimated Fair Value
Less than 12 months, estimated fair value	5,641	23,784
12 months or more, estimated fair value	13,157	33,873
Total estimated fair value	18,798	57,657
Unrealized Losses
Less than 12 months, unrealized losses	(3)	(52)
12 months or more, unrealized losses	(37)	(252)
Total unrealized losses	(40)	(304)
Corporate debt securities
Estimated Fair Value
Less than 12 months, estimated fair value	9,168	17,291
12 months or more, estimated fair value	8,049	17,952
Total estimated fair value	17,217	35,243
Unrealized Losses
Less than 12 months, unrealized losses	(60)	(111)
12 months or more, unrealized losses	(101)	(416)
Total unrealized losses	(161)	(527)
Mortgage-backed securities
Estimated Fair Value
Less than 12 months, estimated fair value	78,426	119,745
12 months or more, estimated fair value	478,497	435,749
Total estimated fair value	556,923	555,494
Unrealized Losses
Less than 12 months, unrealized losses	(174)	(580)
12 months or more, unrealized losses	(8,933)	(11,399)
Total unrealized losses	$ (9,107)	$ (11,979)