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, 2019	Dec. 31, 2018
Available-for-Sale Debt Securities
Amortized Cost	$ 1,867,358	$ 1,224,227
Gross Unrealized Gain	42,942	3,902
Gross Unrealized Loss	1,479	23,050
Investment securities available for sale	1,908,821	1,205,079
Held-to-Maturity Debt Securities
Debt Securities, Held-to-maturity, Total	134,961	149,950
Gross Unrealized Gain	3,301
Gross Unrealized Loss	0
Fair Value	138,262	149,431
Equity Investments
Amortized Cost	23,865	17,218
Gross Unrealized Gain	46,190	20,015
Gross Unrealized Loss	0	0
Fair Value	70,055	37,233
CMO
Available-for-Sale Debt Securities
Amortized Cost	358,826	376,867
Gross Unrealized Gain	6,550	1,721
Gross Unrealized Loss	130	6,838
Investment securities available for sale	365,246	371,750
FNMA MBS
Available-for-Sale Debt Securities
Amortized Cost	1,136,645	655,485
Gross Unrealized Gain	27,118	1,526
Gross Unrealized Loss	1,016	12,938
Investment securities available for sale	1,162,747	644,073
FHLMC MBS
Available-for-Sale Debt Securities
Amortized Cost	338,326	155,758
Gross Unrealized Gain	8,690	558
Gross Unrealized Loss	261	2,394
Investment securities available for sale	346,755	153,922
GNMA MBS
Available-for-Sale Debt Securities
Amortized Cost	33,561	36,117
Gross Unrealized Gain	584	97
Gross Unrealized Loss	72	880
Investment securities available for sale	34,073	35,334
Other equity investments
Equity Investments
Amortized Cost	8,149	3,300
Gross Unrealized Gain	625	0
Gross Unrealized Loss	0	0
Fair Value	8,774	3,300
State and political subdivisions
Held-to-Maturity Debt Securities
Debt Securities, Held-to-maturity, Total	132,959	149,950
Gross Unrealized Gain	3,298	275
Gross Unrealized Loss	0	794
Fair Value	136,257	149,431
Equity Investments
Available for sale securities transfers to held to maturity unrealized gains	700	1,000
Foreign bonds
Held-to-Maturity Debt Securities
Debt Securities, Held-to-maturity, Total	2,002
Gross Unrealized Gain	3
Gross Unrealized Loss	0
Fair Value	2,005
Visa Class B shares | Common Stock
Equity Investments
Amortized Cost	15,716	13,918
Gross Unrealized Gain	45,565	20,015
Gross Unrealized Loss	0	0
Fair Value	$ 61,281	$ 33,933