The amortized cost, estimated fair values and allowance for credit losses of investments in debt securities are summarized in the following tables:


	March 31, 2024
(in thousands)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Allowance for Credit Losses		Estimated Fair Value
Debt Securities Available for Sale
Obligations of U.S. government agencies	$	1,349,679	$	1	$	(179,488)	$	—	$	1,170,192
Obligations of states and political subdivisions	262,777		132		(28,732)		—		234,177
Corporate bonds	6,175		—		(429)		—		5,746
Asset backed securities	362,877		623		(3,827)		—		359,673
Non-agency collateralized mortgage obligations	340,100		—		(36,000)		—		304,100
Total debt securities available for sale	$	2,321,608	$	756	$	(248,476)	$	—	$	2,073,888
Debt Securities Held to Maturity
Obligations of U.S. government agencies	$	125,131	$	1	$	(9,239)	—		115,893
Obligations of states and political subdivisions	2,680		—		(45)		—		2,635
Total debt securities held to maturity	$	127,811	$	1	$	(9,284)	$	—	$	118,528


	December 31, 2023
(in thousands)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Allowance for Credit Losses		Estimated Fair Value
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

There were no sales of investment securities during the three months ended March 31, 2024. Proceeds from the sale of investment securities totaled $24.2 million for the three months ended March 31, 2023, resulting in gross realized losses of $0.2 million. Investment securities with an aggregate carrying value of $741.5 million and $702.2 million at March 31, 2024 and December 31, 2023, respectively, were pledged as collateral for specific borrowings, lines of credit or local agency deposits.

The amortized cost and estimated fair value of debt securities at March 31, 2024 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 March 31, 2024, obligations of U.S. government corporations and agencies with a cost basis totaling $1.3 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 March 31, 2024, the Company estimates the average remaining life of these mortgage-backed securities issued by U.S. government corporations and agencies to be approximately 7.25 years. Average remaining life is defined as the time span after which the principal balance has been reduced by half.


Debt Securities	Available for Sale				Held to Maturity
(in thousands)	Amortized Cost		Estimated Fair Value		Amortized Cost		Estimated Fair Value

Due in one year	$	51,718	$	51,226	$	—	$	—
Due after one year through five years	49,150		46,062		5,396		5,206
Due after five years through ten years	371,852		358,110		92,951		86,475
Due after ten years	1,848,888		1,618,490		29,464		26,847
Totals	$	2,321,608	$	2,073,888	$	127,811	$	118,528

Gross unrealized losses on debt 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:


March 31, 2024:	Less than 12 months				12 months or more				Total
(in thousands)	Fair Value		Unrealized Loss		Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
Debt Securities Available for Sale
Obligations of U.S. government agencies	$	442	$	(3)	$	1,169,687	$	(179,485)	$	1,170,129	$	(179,488)
Obligations of states and political subdivisions	6,760		(152)		219,908		(28,580)		226,668		(28,732)
Corporate bonds	—		—		5,746		(429)		5,746		(429)
Asset backed securities	94,861		(350)		146,838		(3,477)		241,699		(3,827)
Non-agency collateralized mortgage obligations	43,223		(523)		260,877		(35,477)		304,100		(36,000)
Total debt securities available for sale	$	145,286	$	(1,028)	$	1,803,056	$	(247,448)	$	1,948,342	$	(248,476)
Debt Securities Held to Maturity
Obligations of U.S. government agencies	$	—	$	—	$	115,660	$	(9,239)	$	115,660	$	(9,239)
Obligations of states and political subdivisions	1,618		(8)		1,018		(37)		2,636		(45)
Total debt securities held to maturity	$	1,618	$	(8)	$	116,678	$	(9,276)	$	118,296	$	(9,284)


December 31, 2023:	Less than 12 months				12 months or more				Total
(in thousands)	Fair Value		Unrealized Loss		Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
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)

Obligations of U.S. government agencies: The unrealized losses on investments in obligations of U.S. government agencies are caused by interest rate increases and illiquidity. 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 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 recorded. At March 31, 2024, 168 debt securities representing obligations of U.S. government agencies had unrealized losses with aggregate depreciation of 13.29% 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 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 recorded as of March 31, 2024. At March 31, 2024, 157 debt securities representing obligations of states and political subdivisions had unrealized losses with aggregate depreciation of 11.25% 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 recorded as of March 31, 2024. At March 31, 2024, 6 debt securities representing corporate bonds had unrealized losses with aggregate depreciation of 6.95% 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 for these types of securities. At the time of purchase, each of these securities was rated AA or AAA and through March 31, 2024

has not experienced any deterioration in credit rating. At March 31, 2024, 29 asset backed securities had unrealized losses with aggregate depreciation of 1.56% from the Company’s amortized cost basis. 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, there is no impairment on these securities and there has been no allowance for credit losses recorded as of March 31, 2024.

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 March 31, 2024. At March 31, 2024, 22 asset backed securities had unrealized losses with aggregate depreciation of 10.59% from the Company’s amortized cost basis.

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:


	March 31, 2024				December 31, 2023
(in thousands)	AAA/AA/A		BBB/BB/B		AAA/AA/A		BBB/BB/B
Obligations of U.S. government agencies	$	125,131	$	—	$	130,823	$	—
Obligations of states and political subdivisions	2,680		—		2,671		—
Total debt securities held to maturity	$	127,811	$	—	$	133,494	$	—