INVESTMENTS - Schedule of Amortized Cost and Estimated Fair Value of Available-for-Sale and Held-to-Maturity Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Available-for-Sale Debt Securities
Amortized Cost	$ 1,953,089	$ 1,909,483
Gross Unrealized Gain	95,364	39,413
Gross Unrealized Loss	53	3,982
Allowance for Credit Losses	0
Investment securities available-for-sale	2,048,400	1,944,914
Held-to-Maturity Debt Securities
Debt Securities, Held-to-maturity, Total	134,055	133,601
Gross Unrealized Gain	3,327	3,024
Gross Unrealized Loss	0	0
Allowance for credit loss	8	136,625
Fair Value	137,374	136,625
State and political subdivisions
Held-to-Maturity Debt Securities
Debt Securities, Held-to-maturity, Total	133,554	131,600
Gross Unrealized Gain	3,327	3,023
Gross Unrealized Loss	0	0
Allowance for credit loss	8	0
Fair Value	136,873	134,623
Available for sale securities transfers to held to maturity unrealized gains	500	(600)
Foreign bonds
Held-to-Maturity Debt Securities
Debt Securities, Held-to-maturity, Total	501	2,001
Gross Unrealized Gain	0	1
Gross Unrealized Loss	0	0
Allowance for credit loss	0	2,002
Fair Value	501
CMO
Available-for-Sale Debt Securities
Amortized Cost	318,881	336,194
Gross Unrealized Gain	14,564	4,578
Gross Unrealized Loss	47	542
Allowance for Credit Losses	0
Investment securities available-for-sale	333,398	340,230
FNMA MBS
Available-for-Sale Debt Securities
Amortized Cost	1,298,745	1,219,522
Gross Unrealized Gain	62,925	25,717
Gross Unrealized Loss	6	2,786
Allowance for Credit Losses	0
Investment securities available-for-sale	1,361,664	1,242,453
FHLMC MBS
Available-for-Sale Debt Securities
Amortized Cost	303,400	320,896
Gross Unrealized Gain	16,646	8,641
Gross Unrealized Loss	0	591
Allowance for Credit Losses	0
Investment securities available-for-sale	320,046	328,946
GNMA MBS
Available-for-Sale Debt Securities
Amortized Cost	32,063	32,871
Gross Unrealized Gain	1,229	477
Gross Unrealized Loss	0	63
Allowance for Credit Losses	0
Investment securities available-for-sale	$ 33,292	$ 33,285

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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