INVESTMENT SECURITIES - Schedule of Amortized Cost and Estimated Fair Value of Available-for-Sale and Held-to-Maturity Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Available-for-Sale Debt Securities
Amortized Cost	$ 1,036,905	$ 847,791
Gross Unrealized Gain	74	1,356
Gross Unrealized Loss	39,848	11,648
Fair Value	997,131	837,499
Held-to-Maturity Debt Securities
Amortized Cost	152,577	161,186
Fair Value	150,583	162,853
Equity Investments
Amortized Cost	17,218	643
Gross Unrealized Gain	17,865	0
Gross Unrealized Loss	0	20
Fair Value	35,083	623
CMO
Available-for-Sale Debt Securities
Amortized Cost	311,507	250,592
Gross Unrealized Gain	0	88
Gross Unrealized Loss	11,477	4,141
Fair Value	300,030	246,539
FNMA MBS
Available-for-Sale Debt Securities
Amortized Cost	571,686	479,218
Gross Unrealized Gain	0	941
Gross Unrealized Loss	22,948	6,172
Fair Value	548,738	473,987
FHLMC MBS
Available-for-Sale Debt Securities
Amortized Cost	116,532	88,681
Gross Unrealized Gain	2	118
Gross Unrealized Loss	4,112	924
Fair Value	112,422	87,875
GNMA MBS
Available-for-Sale Debt Securities
Amortized Cost	37,180	29,300
Gross Unrealized Gain	72	209
Gross Unrealized Loss	1,311	411
Fair Value	35,941	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	152,577	161,186
Gross Unrealized Gain	55	1,758
Gross Unrealized Loss	2,049	91
Fair Value	150,583	162,853
Equity Investments
Available for sale securities transfers to held to maturity unrealized gains	1,200	$ 1,600
Visa | Visa Class B shares | Common Stock
Equity Investments
Amortized Cost	13,918
Gross Unrealized Gain	17,865
Gross Unrealized Loss	0
Fair Value	$ 31,783