INVESTMENT SECURITIES - Schedule of Amortized Cost and Estimated Fair Value of Available-for-Sale and Held-to-Maturity Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Available-for-Sale Debt Securities
Amortized Cost	$ 995,943	$ 847,791
Gross Unrealized Gain	166	1,356
Gross Unrealized Loss	31,989	11,648
Fair Value	964,120	837,499
Held-to-Maturity Debt Securities
Amortized Cost	156,456	161,186
Fair Value	155,644	162,853
Equity Investments
Amortized Cost	18,243	643
Gross Unrealized Gain	15,346	0
Gross Unrealized Loss	0	20
Fair Value	33,589	623
CMO
Available-for-Sale Debt Securities
Amortized Cost	307,500	250,592
Gross Unrealized Gain	61	88
Gross Unrealized Loss	9,298	4,141
Fair Value	298,263	246,539
FNMA MBS
Available-for-Sale Debt Securities
Amortized Cost	556,567	479,218
Gross Unrealized Gain	9	941
Gross Unrealized Loss	18,336	6,172
Fair Value	538,240	473,987
FHLMC MBS
Available-for-Sale Debt Securities
Amortized Cost	97,033	88,681
Gross Unrealized Gain	2	118
Gross Unrealized Loss	3,297	924
Fair Value	93,738	87,875
GNMA MBS
Available-for-Sale Debt Securities
Amortized Cost	34,843	29,300
Gross Unrealized Gain	94	209
Gross Unrealized Loss	1,058	411
Fair Value	33,879	29,098
Other equity investments
Equity Investments
Amortized Cost	3,300	643
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	20
Fair Value	3,300	623
State and political subdivisions
Held-to-Maturity Debt Securities
Amortized Cost	156,456	161,186
Gross Unrealized Gain	231	1,758
Gross Unrealized Loss	1,043	91
Fair Value	155,644	162,853
Equity Investments
Available for sale securities transfers to held to maturity unrealized gains	1,300	$ 1,600
Visa | Visa Class B shares | Common Stock
Equity Investments
Amortized Cost	14,943
Gross Unrealized Gain	15,346
Gross Unrealized Loss	0
Fair Value	$ 30,289