Investment Securities - Schedule of Amortized Cost and Estimated Fair Value of Available-for-Sale and Held-to-Maturity Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Available-for-Sale Debt Securities
Amortized cost of investment securities, available for sale	$ 2,450,265	$ 1,909,483
Gross Unrealized Gain	80,871	39,413
Gross Unrealized Loss	2,079	3,982
Allowance for Credit Losses	0
Available-for-sale securities	2,529,057	1,944,914
Held-to-Maturity Debt Securities
Investment securities, held to maturity, net	111,747	133,601
Gross Unrealized Gain	4,680	3,024
Allowance for credit loss	6
Gross Unrealized Loss	0	0
Fair Value	116,421	136,625
State and political subdivisions
Held-to-Maturity Debt Securities
Investment securities, held to maturity, net	111,246	131,600
Gross Unrealized Gain	4,678	3,023
Allowance for credit loss	6	0
Gross Unrealized Loss	0	0
Fair Value	115,918	134,623
Available for sale securities transfers to held to maturity unrealized gains	400	600
Foreign bonds
Held-to-Maturity Debt Securities
Investment securities, held to maturity, net	501	2,001
Gross Unrealized Gain	2	1
Allowance for credit loss	0
Gross Unrealized Loss	0	0
Fair Value	503	2,002
CMO
Available-for-Sale Debt Securities
Amortized cost of investment securities, available for sale	461,819	336,194
Gross Unrealized Gain	9,949	4,578
Gross Unrealized Loss	443	542
Allowance for Credit Losses	0
Available-for-sale securities	471,325	340,230
FNMA MBS
Available-for-Sale Debt Securities
Amortized cost of investment securities, available for sale	1,544,105	1,219,522
Gross Unrealized Gain	55,747	25,717
Gross Unrealized Loss	882	2,786
Allowance for Credit Losses	0
Available-for-sale securities	1,598,970	1,242,453
FHLMC MBS
Available-for-Sale Debt Securities
Amortized cost of investment securities, available for sale	190,856	320,896
Gross Unrealized Gain	12,142	8,641
Gross Unrealized Loss	105	591
Allowance for Credit Losses	0
Available-for-sale securities	202,893	328,946
GNMA MBS
Available-for-Sale Debt Securities
Amortized cost of investment securities, available for sale	22,716	32,871
Gross Unrealized Gain	1,046	477
Gross Unrealized Loss	0	63
Allowance for Credit Losses	0
Available-for-sale securities	23,762	$ 33,285
GSE agency notes
Available-for-Sale Debt Securities
Amortized cost of investment securities, available for sale	230,769
Gross Unrealized Gain	1,987
Gross Unrealized Loss	649
Allowance for Credit Losses	0
Available-for-sale securities	$ 232,107

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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