The following tables detail the amortized cost, allowance for credit losses and the estimated fair value of the Company's investments in available-for-sale and held-to-maturity debt securities. None of the Company's investments in debt securities are classified as trading.


	March 31, 2024
(Dollars in thousands)	Amortized Cost		Gross Unrealized Gain		Gross Unrealized Loss		Allowance for Credit Losses		Fair Value
Available-for-Sale Debt Securities
Collateralized mortgage obligation (CMO)	$	551,883	$	—	$	99,992	$	—	$	451,891
Fannie Mae (FNMA) mortgage-backed securities (MBS)	3,495,526		118		548,295		—		2,947,349
Freddie Mac (FHLMC) MBS	124,992		—		13,241		—		111,751
Ginnie Mae (GNMA) MBS	47,478		1		3,539		—		43,940
Government-sponsored enterprises (GSE) agency notes	224,797		—		45,499		—		179,298
	$	4,444,676	$	119	$	710,566	$	—	$	3,734,229
Held-to-Maturity Debt Securities(1)
FNMA MBS	$	864,584	$	—	$	99,912	$	—	$	764,672
State and political subdivisions	185,231		1,368		1,536		8		185,055

	$	1,049,815	$	1,368	$	101,448	$	8	$	949,727

(1)Held-to-maturity securities transferred from available-for-sale are included in held-to-maturity at fair value basis at the time of transfer. The amortized cost of transferred held-to-maturity securities included net unrealized losses of $115.6 million at March 31, 2024, which are offset in Accumulated other comprehensive loss. At the time of transfer, there was no allowance for credit loss on the available-for-sale securities. Subsequent to transfer, the securities were evaluated for credit loss.


	December 31, 2023
(Dollars in thousands)	Amortized Cost		Gross Unrealized Gain		Gross Unrealized Loss		Allowance for Credit Losses		Fair Value
Available-for-Sale Debt Securities
CMO	$	560,952	$	—	$	96,333	$	—	$	464,619
FNMA MBS	3,544,762		162		502,574		—		3,042,350
FHLMC MBS	126,856		—		11,324		—		115,532
GNMA MBS	46,333		6		2,999		—		43,340
GSE agency notes	225,439		—		44,743		—		180,696
	$	4,504,342	$	168	$	657,973	$	—	$	3,846,537
Held-to-Maturity Debt Securities(1)
FNMA MBS	$	872,653	$	—	$	74,332	$	—	$	798,321
State and political subdivisions	185,912		2,665		959		8		187,610

	$	1,058,565	$	2,665	$	75,291	$	8	$	985,931

(1)Held-to-maturity securities transferred from available-for-sale are included in held-to-maturity at fair value at the time of transfer. The amortized cost of transferred held-to-maturity securities included net unrealized losses of $120.4 million at December 31, 2023, which are offset in Accumulated other comprehensive loss. At the time of transfer, there was no allowance for credit loss on the available-for-sale securities. Subsequent to transfer, the securities were evaluated for credit loss.

The scheduled maturities of available-for-sale debt securities at March 31, 2024 and December 31, 2023 are presented in the table below:


	Available-for-Sale
	Amortized		Fair
(Dollars in thousands)	Cost		Value
March 31, 2024 (1)
Within one year	$	2,480	$	2,415
After one year but within five years	97,525		92,024
After five years but within ten years	549,047		462,583
After ten years	3,795,624		3,177,207
	$	4,444,676	$	3,734,229
December 31, 2023 (1)
Within one year	$	—	$	—
After one year but within five years	86,224		82,387
After five years but within ten years	569,956		485,593
After ten years	3,848,162		3,278,557
	$	4,504,342	$	3,846,537

As of March 31, 2024, the Company’s available-for-sale investment securities consisted of 973 securities, 965 of which were in an unrealized loss position.

As of March 31, 2024, substantially all of the Corporation’s available-for-sale investment securities were mortgage-backed securities or collateral mortgage obligations which were issued or guaranteed by U.S. government-sponsored entities and agencies. As of March 31, 2024 and December 31, 2023, there were no holdings of securities of any one issuer, other than the U.S. government and its agencies, in an amount greater than 10% of shareholders’ equity.


	Held-to-Maturity
	Amortized		Fair
(Dollars in thousands)	Cost		Value
March 31, 2024 (1)
Within one year	$	—	$	—
After one year but within five years	11,930		11,741
After five years but within ten years	47,195		46,747
After ten years	990,690		891,239
	$	1,049,815	$	949,727
December 31, 2023 (1)
Within one year	$	—	$	—
After one year but within five years	10,932		10,856
After five years but within ten years	46,489		46,246
After ten years	1,001,144		928,829
	$	1,058,565	$	985,931

MBS may have expected maturities that differ from their contractual maturities. These differences arise because issuers may have the right to call securities and borrowers may have the right to prepay obligations with or without prepayment penalty. The estimated weighted average duration of MBS was 5.8 years at March 31, 2024.

The held-to-maturity debt securities are not collateral-dependent securities as these are general obligation bonds issued by cities, states, counties, or other local and foreign governments.

Investment securities with fair market values aggregating $3.0 billion and $3.3 billion were pledged as collateral for investment sweep repurchase agreements, municipal deposits, and other obligations as of March 31, 2024 and December 31, 2023, respectively.

As of March 31, 2024 and December 31, 2023, the Company's debt securities portfolio had remaining unamortized premiums of $54.7 million and $56.9 million, respectively, and unaccreted discounts of $20.3 million and $20.9 million, respectively.

For debt securities in an unrealized loss position, the table below shows the gross unrealized losses and fair value by investment category and length of time that individual debt securities were in a continuous unrealized loss position at March 31, 2024.


	Duration of Unrealized Loss Position
	Less than 12 months				12 months or longer				Total
	Fair		Unrealized		Fair		Unrealized		Fair		Unrealized
(Dollars in thousands)	Value		Loss		Value		Loss		Value		Loss
Available-for-sale debt securities:
CMO	$	—	$	—	$	451,891	$	99,992	$	451,891	$	99,992
FNMA MBS	18,744		275		2,916,614		548,020		2,935,358		548,295
FHLMC MBS	—		—		111,745		13,241		111,745		13,241
GNMA MBS	7,785		227		35,109		3,312		42,894		3,539
GSE agency notes	—		—		179,298		45,499		179,298		45,499
	$	26,529	$	502	$	3,694,657	$	710,064	$	3,721,186	$	710,566


	Duration of Unrealized Loss Position
	Less than 12 months				12 months or longer				Total
	Fair		Unrealized		Fair		Unrealized		Fair		Unrealized
(Dollars in thousands)	Value		Loss		Value		Loss		Value		Loss
Available-for-sale debt securities:
CMO	$	—	$	—	$	464,619	$	96,333	$	464,619	$	96,333
FNMA MBS	9,068		125		3,026,520		502,449		3,035,588		502,574
FHLMC MBS	—		—		115,525		11,324		115,525		11,324
GNMA MBS	10,543		217		31,681		2,782		42,224		2,999
GSE agency notes	—		—		180,696		44,743		180,696		44,743
	$	19,611	$	342	$	3,819,041	$	657,631	$	3,838,652	$	657,973

The Company does not have the intent to sell, nor is it more likely than not it will be required to sell these securities before it is able to recover the amortized cost basis. The unrealized losses are the result of changes in market interest rates subsequent to purchase, not credit loss, as these are highly rated agency securities with no expected credit loss, in the event of a default. As a result, there is no allowance for credit losses recorded for available-for-sale debt securities as of March 31, 2024.

At March 31, 2024 and December 31, 2023, held-to-maturity debt securities had an amortized cost basis of $1.0 billion. The held-to-maturity debt security portfolio primarily consists of mortgage-backed securities which were issued or guaranteed by U.S. government-sponsored entities and agencies and highly rated municipal bonds. The Company monitors credit quality of its non-government and non-agency securities through credit ratings. The following table summarizes the amortized cost of debt securities held-to-maturity as of March 31, 2024, aggregated by credit quality indicator:


(Dollars in thousands)	FNMA MBS		State and political subdivisions
A+ rated or higher	$	—	$	185,231
Not rated	864,584		—
Ending balance	$	864,584	$	185,231


(Dollars in thousands)	FNMA MBS		State and political subdivisions
A+ rated or higher	$	—	$	185,912
Not rated	872,653		—
Ending balance	$	872,653	$	185,912

The Company reviewed its held-to-maturity debt securities by major security type for potential credit losses. There was no activity in the allowance for credit losses for FNMA MBS and foreign bond debt securities for the three months ended March 31, 2024 and 2023. The following table presents the activity in the allowance for credit losses for state and political subdivisions debt securities for the three months ended March 31, 2024 and 2023:


	Three months ended March 31,
(Dollars in thousands)	2024		2023
Allowance for credit losses:
Beginning balance	$	8	$	10

Provision for (recovery of) credit losses	—		(1)



Ending balance	$	8	$	9

Accrued interest receivable of $3.2 million and $3.7 million as of March 31, 2024 and December 31, 2023, respectively, for held-to-maturity debt securities were excluded from the evaluation of allowance for credit losses. There were no nonaccrual or past due held-to-maturity debt securities as of March 31, 2024 and December 31, 2023.

During the three months ended March 31, 2023, the Company recognized $1.3 million of net losses related to our equity method investments within Other income on the unaudited Consolidated Statements of Income.