The amortized cost and estimated fair values of investment securities classified as available for sale and held to maturity are summarized in the following tables:


	December 31, 2023
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
			(in thousands)
Debt Securities Available for Sale
Obligations of U.S. government agencies	$	1,386,772	$	2	$	(165,037)	$	1,221,737
Obligations of states and political subdivisions	262,879		268		(26,772)		236,375
Corporate bonds	6,173		—		(571)		5,602
Asset backed securities	359,214		255		(4,188)		355,281
Non-agency collateralized mortgage obligations	369,287		—		(35,778)		333,509
Total debt securities available for sale	$	2,384,325	$	525	$	(232,346)	$	2,152,504
Debt Securities Held to Maturity
Obligations of U.S. government agencies	$	130,823	$	—	$	(8,331)	$	122,492
Obligations of states and political subdivisions	2,671		6		(43)		2,634
Total debt securities held to maturity	$	133,494	$	6	$	(8,374)	$	125,126


	December 31, 2022
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
			(in thousands)
Debt Securities Available for Sale
Obligations of U.S. government agencies	$	1,568,408	$	3	$	(195,642)	$	1,372,769
Obligations of states and political subdivisions	332,625		401		(39,821)		293,205
Corporate bonds	6,164		—		(413)		5,751
Asset backed securities	454,943		17		(15,193)		439,767
Non-agency collateralized mortgage obligations	380,847		—		(39,901)		340,946
Total debt securities available for sale	$	2,742,987	$	421	$	(290,970)	$	2,452,438
Debt Securities Held to Maturity
Obligations of U.S. government agencies	$	154,830	$	2	$	(11,013)	$	143,819
Obligations of states and political subdivisions	6,153		13		(47)		6,119
Total debt securities held to maturity	$	160,983	$	15	$	(11,060)	$	149,938

During the year ended December 31, 2023, proceeds from sales of debt securities totaled $71.0 million, resulting in gross losses of $0.3 million. There were no sales of debt securities during the years ended 2022 and 2021, respectively. Investment securities with an aggregate carrying value of $702.2 million and $595.8 million at December 31, 2023 and 2022, respectively, were pledged as collateral for specific borrowings, lines of credit and local agency deposits.

The amortized cost and estimated fair value of debt securities at December 31, 2023 by contractual maturity are shown below. Actual maturities may differ from contractual maturities because borrowers may have the right to call or prepay obligations with or without call or prepayment penalties. At December 31, 2023, obligations of U.S. government and agencies with an amortized cost basis totaling $1.4 billion consist almost entirely of residential real estate mortgage-backed securities whose contractual maturity, or principal repayment, will follow the repayment of the underlying mortgages. For purposes of the following table, the entire outstanding balance of these mortgage-backed securities issued by U.S. government corporations and agencies is categorized based on final maturity date. At December 31, 2023, the Company estimates the average remaining life of these mortgage-backed securities issued by U.S. government corporations and agencies to be approximately 6.7 years. Average remaining life is defined as the time span after which the principal balance has been reduced by half.


December 31, 2023	Available for Sale				Held to Maturity
(In thousands)	Amortized Cost		Estimated Fair Value		Amortized Cost		Estimated Fair Value
Due in one year	$	83,239	$	81,775	$	—	$	—
Due after one year through five years	47,769		44,932		5,829		5,640
Due after five years through ten years	367,733		354,333		89,070		83,726
Due after ten years	1,885,584		1,671,464		38,595		35,760
Totals	$	2,384,325	$	2,152,504	$	133,494	$	125,126

Gross unrealized losses on investment securities and the fair value of the related securities, aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position, were as follows:


	Less than 12 months				12 months or more				Total
	Fair Value		Unrealized Loss		Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
December 31, 2023	(in thousands)
Debt Securities Available for Sale
Obligations of U.S. government agencies	$	224	$	—	$	1,221,320	$	(165,037)	$	1,221,544	$	(165,037)
Obligations of states and political subdivisions	6,229		(75)		216,497		(26,697)		222,726		(26,772)
Corporate bonds	—		—		5,602		(571)		5,602		(571)
Asset backed securities	15,928		(93)		264,731		(4,095)		280,659		(4,188)
Non-agency collateralized mortgage obligations	44,276		(583)		289,233		(35,195)		333,509		(35,778)
Total debt securities available for sale	$	66,657	$	(751)	$	1,997,383	$	(231,595)	$	2,064,040	$	(232,346)
Debt Securities Held to Maturity
Obligations of U.S. government agencies	$	—	$	—	$	122,259	$	(8,331)	$	122,259	$	(8,331)
Obligations of states and political subdivisions	—		—		1,012		(43)		1,012		(43)
Total debt securities held to maturity	$	—	$	—	$	123,271	$	(8,374)	$	123,271	$	(8,374)


	Less than 12 months				12 months or more				Total
	Fair Value		Unrealized Loss		Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
December 31, 2022	(in thousands)
Debt Securities Available for Sale
Obligations of U.S. government agencies	$	766,612	$	(134,234)	$	605,615	$	(61,408)	$	1,372,227	$	(195,642)
Obligations of states and political subdivisions	43,282		(12,917)		219,532		(26,904)		262,814		(39,821)
Corporate bonds	—		—		5,751		(413)		5,751		(413)
Asset backed securities	205,329		(10,238)		231,703		(4,955)		437,032		(15,193)
Non-agency collateralized mortgage obligations	203,620		(36,480)		123,075		(3,421)		326,695		(39,901)
Total securities available for sale	$	1,218,843	$	(193,869)	$	1,185,676	$	(97,101)	$	2,404,519	$	(290,970)
Debt Securities Held to Maturity
Obligations of U.S. government agencies	$	—	$	—	$	143,577	$	(11,013)	$	143,577	$	(11,013)
Obligations of states and political subdivisions	—		—		4,530		(47)		4,530		(47)
Total debt securities held to maturity	$	—	$	—	$	148,107	$	(11,060)	$	148,107	$	(11,060)

Obligations of U.S. government agencies: Unrealized losses on investments in obligations of U.S. government corporations and agencies are caused by interest rate increases. The contractual cash flows of these securities are guaranteed by U.S. Government Sponsored Entities (principally Fannie Mae and Freddie Mac). It is expected that the securities would not be settled at a price less than the amortized cost of the investment. Because the decline in fair value is attributable to changes in interest rates and not credit quality, and because the Company does not intend to sell and more likely than not will not be required to sell, these investments are not considered other-than-temporarily impaired. At December 31, 2023, 165 debt securities representing obligations of U.S. government corporations and agencies had unrealized losses with aggregate depreciation of 11.90% from the Company’s amortized cost basis.

Obligations of states and political subdivisions: The unrealized losses on investments in obligations of states and political subdivisions were caused by increases in required yields by investors in these types of securities. It is expected that the securities would not be settled at a

price less than the amortized cost of the investment. Because the decline in fair value is attributable to changes in interest rates and not credit quality, and because the Company does not intend to sell and more likely than not will not be required to sell, these investments are not considered other-than-temporarily impaired. At December 31, 2023, 153 debt security representing obligations of states and political subdivisions had unrealized losses with aggregate depreciation of 10.73% from the Company’s amortized cost basis.

Corporate bonds: The unrealized losses on investments in corporate bonds were caused by increases in required yields by investors in these types of securities. It is expected that the securities would not be settled at a price less than the amortized cost of the investment. Because management believes the decline in fair value is attributable to changes in interest rates and not credit quality, and because the Company does not intend to sell and more likely than not will not be required to sell, there is no impairment on these securities and there has been no allowance for credit losses as of and for the year ended December 31, 2023. At December 31, 2023, 6 asset backed securities had unrealized losses with aggregate depreciation of 9.25% from the Company’s amortized cost basis.

Asset backed securities: The unrealized losses on investments in asset backed securities were caused by increases in required yields by investors in these types of securities. At the time of purchase, each of these securities were rated AA or AAA and through December 31, 2023 have not experienced any deterioration in credit rating. The Company continues to monitor these securities for changes in credit rating or other indications of credit deterioration. Because management believes the decline in fair value is attributable to changes in interest rates and not credit quality, and because the Company does not intend to sell and more likely than not will not be required to sell, these investments are not considered other-than-temporarily impaired. At December 31, 2023, 35 asset backed securities had unrealized losses with aggregate depreciation of 1.47% from the Company’s amortized cost basis.

Non-agency collateralized mortgage obligations: The unrealized losses on investments in asset backed securities were caused by increases in required yields by investors in these types of securities. It is expected that the securities would not be settled at a price less than the amortized cost of the investment. Because management believes the decline in fair value is attributable to changes in interest rates and not credit quality, and because the Company does not intend to sell and more likely than not will not be required to sell, there is no impairment on these securities and there has been no allowance for credit losses as of and for the year ended December 31, 2023. At December 31, 2023, 24 asset backed securities had unrealized losses with aggregate depreciation of 9.69% from the Company’s amortized cost basis.

Marketable equity securities: As there were no sales of marketable equity securities, all unrealized gains or losses recognized during the reporting period were for equity securities still held as of the end of the reporting period.

The Company monitors credit quality of debt securities held-to-maturity through the use of credit rating. The Company monitors the credit rating on a monthly basis. The following table summarizes the amortized cost of debt securities held-to-maturity at the dates indicated, aggregated by credit quality indicator:


	December 31, 2023				December 31, 2022
	AAA/AA/A		BBB/BB/B		AAA/AA/A		BBB/BB/B
	(In thousands)				(In thousands)
Debt Securities Held to Maturity
Obligations of U.S. government agencies	$	130,823	$	—	$	154,830	$	—
Obligations of states and political subdivisions	2,671		—		6,153		—
Total debt securities held to maturity	$	133,494	$	—	$	160,983	$	—