INVESTMENT SECURITIES - Schedule of Gross Unrealized Losses and Fair Value of Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Estimated Fair Value
Less than 12 months, estimated fair value	$ 347,480	$ 160,820
12 months or more, estimated fair value	83,129	487,574
Total estimated fair value	430,609	648,394
Unrealized Losses
Less than 12 months, unrealized losses	(1,858)	(743)
12 months or more, unrealized losses	(1,217)	(12,067)
Total unrealized losses	(3,075)	(12,810)
State, county and municipal securities
Estimated Fair Value
Less than 12 months, estimated fair value	559	23,784
12 months or more, estimated fair value	0	33,873
Total estimated fair value	559	57,657
Unrealized Losses
Less than 12 months, unrealized losses	0	(52)
12 months or more, unrealized losses	0	(252)
Total unrealized losses	0	(304)
Corporate debt securities
Estimated Fair Value
Less than 12 months, estimated fair value	1,470	17,291
12 months or more, estimated fair value	2,090	17,952
Total estimated fair value	3,560	35,243
Unrealized Losses
Less than 12 months, unrealized losses	(30)	(111)
12 months or more, unrealized losses	(3)	(416)
Total unrealized losses	(33)	(527)
Mortgage-backed securities
Estimated Fair Value
Less than 12 months, estimated fair value	345,451	119,745
12 months or more, estimated fair value	81,039	435,749
Total estimated fair value	426,490	555,494
Unrealized Losses
Less than 12 months, unrealized losses	(1,828)	(580)
12 months or more, unrealized losses	(1,214)	(11,399)
Total unrealized losses	$ (3,042)	$ (11,979)