INVESTMENT SECURITIES - Schedule of Gross Unrealized Losses and Fair Value of Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Estimated Fair Value
Less than 12 months, estimated fair value	$ 679,198	$ 299,794
12 months or more, estimated fair value	297,115	213,946
Total estimated fair value	976,313	513,740
Unrealized Losses
Less than 12 months, unrealized losses	(10,145)	(2,551)
12 months or more, unrealized losses	(13,247)	(4,341)
Total unrealized losses	(23,392)	(6,892)
State, county and municipal securities
Estimated Fair Value
Less than 12 months, estimated fair value	83,727	33,976
12 months or more, estimated fair value	12,067	4,725
Total estimated fair value	95,794	38,701
Unrealized Losses
Less than 12 months, unrealized losses	(688)	(115)
12 months or more, unrealized losses	(193)	(48)
Total unrealized losses	(881)	(163)
Corporate debt securities
Estimated Fair Value
Less than 12 months, estimated fair value	12,141	3,465
12 months or more, estimated fair value	17,970	18,853
Total estimated fair value	30,111	22,318
Unrealized Losses
Less than 12 months, unrealized losses	(113)	(35)
12 months or more, unrealized losses	(446)	(202)
Total unrealized losses	(559)	(237)
Mortgage-backed securities
Estimated Fair Value
Less than 12 months, estimated fair value	583,330	262,353
12 months or more, estimated fair value	267,078	190,368
Total estimated fair value	850,408	452,721
Unrealized Losses
Less than 12 months, unrealized losses	(9,344)	(2,401)
12 months or more, unrealized losses	(12,608)	(4,091)
Total unrealized losses	$ (21,952)	$ (6,492)