INVESTMENTS - Schedule of Amortized Cost and Estimated Fair Value of Available-for-Sale and Held-to-Maturity Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Available-for-Sale Debt Securities
Available-for-sale securities, amortized cost basis	$ 2,097,114	$ 1,909,483
Gross Unrealized Gain	98,466	39,413
Gross Unrealized Loss	191	3,982
Allowance for Credit Losses	0
Investment securities available-for-sale	2,195,389	1,944,914
Held-to-Maturity Debt Securities
Investment securities held-to-maturity, net	127,609	133,601
Gross Unrealized Gain	4,597	3,024
Gross Unrealized Loss	0	0
Allowance for credit loss	8	136,625
Fair Value	132,198	136,625
State and political subdivisions
Held-to-Maturity Debt Securities
Investment securities held-to-maturity, net	127,108	131,600
Gross Unrealized Gain	4,597	3,023
Gross Unrealized Loss	0	0
Allowance for credit loss	8	0
Fair Value	131,697	134,623
Available for sale securities transfers to held to maturity unrealized gains	500	600
Foreign bonds
Held-to-Maturity Debt Securities
Investment securities held-to-maturity, net	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
Available-for-sale securities, amortized cost basis	343,725	336,194
Gross Unrealized Gain	12,185	4,578
Gross Unrealized Loss	89	542
Allowance for Credit Losses	0
Investment securities available-for-sale	355,821	340,230
FNMA MBS
Available-for-Sale Debt Securities
Available-for-sale securities, amortized cost basis	1,326,940	1,219,522
Gross Unrealized Gain	67,158	25,717
Gross Unrealized Loss	63	2,786
Allowance for Credit Losses	0
Investment securities available-for-sale	1,394,035	1,242,453
FHLMC MBS
Available-for-Sale Debt Securities
Available-for-sale securities, amortized cost basis	280,752	320,896
Gross Unrealized Gain	16,795	8,641
Gross Unrealized Loss	0	591
Allowance for Credit Losses	0
Investment securities available-for-sale	297,547	328,946
GNMA MBS
Available-for-Sale Debt Securities
Available-for-sale securities, amortized cost basis	29,567	32,871
Gross Unrealized Gain	1,257	477
Gross Unrealized Loss	0	63
Allowance for Credit Losses	0
Investment securities available-for-sale	30,824	$ 33,285
GSE
Available-for-Sale Debt Securities
Available-for-sale securities, amortized cost basis	116,130
Gross Unrealized Gain	1,071
Gross Unrealized Loss	39
Allowance for Credit Losses	0
Investment securities available-for-sale	$ 117,162

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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