INVESTMENT SECURITIES - Schedule of Gross Unrealized Losses and Fair Value of Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Estimated Fair Value
Less than 12 months, estimated fair value	$ 717,627	$ 299,794
12 months or more, estimated fair value	222,647	213,946
Total estimated fair value	940,274	513,740
Unrealized Losses
Less than 12 months, unrealized losses	(9,550)	(2,551)
12 months or more, unrealized losses	(9,296)	(4,341)
Total unrealized losses	(18,846)	(6,892)
State, county and municipal securities
Estimated Fair Value
Less than 12 months, estimated fair value	82,129	33,976
12 months or more, estimated fair value	4,627	4,725
Total estimated fair value	86,756	38,701
Unrealized Losses
Less than 12 months, unrealized losses	(463)	(115)
12 months or more, unrealized losses	(96)	(48)
Total unrealized losses	(559)	(163)
Corporate debt securities
Estimated Fair Value
Less than 12 months, estimated fair value	2,475	3,465
12 months or more, estimated fair value	17,979	18,853
Total estimated fair value	20,454	22,318
Unrealized Losses
Less than 12 months, unrealized losses	(148)	(35)
12 months or more, unrealized losses	(483)	(202)
Total unrealized losses	(631)	(237)
Mortgage-backed securities
Estimated Fair Value
Less than 12 months, estimated fair value	633,023	262,353
12 months or more, estimated fair value	200,041	190,368
Total estimated fair value	833,064	452,721
Unrealized Losses
Less than 12 months, unrealized losses	(8,939)	(2,401)
12 months or more, unrealized losses	(8,717)	(4,091)
Total unrealized losses	$ (17,656)	$ (6,492)