INVESTMENT SECURITIES - Schedule of Amortized Cost and Estimated Fair Value of Available-for-Sale and Held-to-Maturity Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Available-for-Sale Debt Securities
Amortized Cost	$ 1,519,643	$ 1,224,227
Gross Unrealized Gain	12,927	3,902
Gross Unrealized Loss	9,374	23,050
Fair Value	1,523,196	1,205,079
Held-to-Maturity Debt Securities
Amortized Cost	148,190	149,950
Gross Unrealized Gain	1,593
Gross Unrealized Loss	51
Fair Value	149,732	149,431
Equity Investments
Amortized Cost	24,638	17,218
Gross Unrealized Gain	23,812	20,015
Gross Unrealized Loss	0	0
Fair Value	48,450	37,233
CMO
Available-for-Sale Debt Securities
Amortized Cost	395,818	376,867
Gross Unrealized Gain	2,807	1,721
Gross Unrealized Loss	3,445	6,838
Fair Value	395,180	371,750
FNMA MBS
Available-for-Sale Debt Securities
Amortized Cost	875,017	655,485
Gross Unrealized Gain	7,490	1,526
Gross Unrealized Loss	4,734	12,938
Fair Value	877,773	644,073
FHLMC MBS
Available-for-Sale Debt Securities
Amortized Cost	213,248	155,758
Gross Unrealized Gain	2,431	558
Gross Unrealized Loss	747	2,394
Fair Value	214,932	153,922
GNMA MBS
Available-for-Sale Debt Securities
Amortized Cost	35,560	36,117
Gross Unrealized Gain	199	97
Gross Unrealized Loss	448	880
Fair Value	35,311	35,334
Other equity investments
Equity Investments
Amortized Cost	8,922	3,300
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	0
Fair Value	8,922	3,300
State and political subdivisions
Held-to-Maturity Debt Securities
Amortized Cost	146,188	149,950
Gross Unrealized Gain	1,592	275
Gross Unrealized Loss	51	794
Fair Value	147,729	149,431
Equity Investments
Available for sale securities transfers to held to maturity unrealized gains	1,000	1,000
Foreign bonds
Held-to-Maturity Debt Securities
Amortized Cost	2,002
Gross Unrealized Gain	1
Gross Unrealized Loss	0
Fair Value	2,003
Visa | Visa Class B shares | Common Stock
Equity Investments
Amortized Cost	15,716	13,918
Gross Unrealized Gain	23,812	20,015
Gross Unrealized Loss	0	0
Fair Value	$ 39,528	$ 33,933