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, 2021
(Dollars in thousands)	Amortized Cost		Gross Unrealized Gain		Gross Unrealized Loss		Allowance for Credit Losses		Fair Value
Available-for-Sale Debt Securities
CMO	$	564,055	$	5,604	$	15,223	$	—	$	554,436
FNMA MBS	1,997,140		35,151		34,246		—		1,998,045
FHLMC MBS	182,271		8,941		759		—		190,453
GNMA MBS	25,011		907		83		—		25,835
GSE agency notes	232,509		—		13,393		—		219,116
	$	3,000,986	$	50,603	$	63,704	$	—	$	2,987,885
Held-to-Maturity Debt Securities(1)
State and political subdivisions	$	103,027	$	4,107	$	—	$	5	$	107,129
Foreign bonds	501		—		1		—		500
	$	103,528	$	4,107	$	1	$	5	$	107,629

(1)Held-to-maturity securities transferred from available-for-sale are included in held-to-maturity at amortized cost basis at the time of transfer. The amortized cost of transferred held-to-maturity securities included net unrealized gains of $0.3 million at March 31, 2021, which are offset in Accumulated other comprehensive (loss) income. 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, 2020
(Dollars in thousands)	Amortized Cost		Gross Unrealized Gain		Gross Unrealized Loss		Allowance for Credit Losses		Fair Value
Available-for-Sale Debt Securities
CMO	$	461,819	$	9,949	$	443	$	—	$	471,325
FNMA MBS	1,544,105		55,747		882		—		1,598,970
FHLMC MBS	190,856		12,142		105		—		202,893
GNMA MBS	22,716		1,046		—		—		23,762
GSE agency notes	230,769		1,987		649		—		232,107
	$	2,450,265	$	80,871	$	2,079	$	—	$	2,529,057
Held-to-Maturity Debt Securities(1)
State and political subdivisions	$	111,246	$	4,678	$	—	$	6	$	115,918
Foreign bonds	501		2		—		—		503
	$	111,747	$	4,680	$	—	$	6	$	116,421

(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 gains of $0.4 million at December 31, 2020, which are offset in Accumulated other comprehensive (loss) income. 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.


	Available-for-Sale
	Amortized		Fair
(Dollars in thousands)	Cost		Value
March 31, 2021 (1)
Within one year	$	—	$	—
After one year but within five years	57,993		60,781
After five years but within ten years	233,748		235,304
After ten years	2,709,245		2,691,800
	$	3,000,986	$	2,987,885
December 31, 2020 (1)
Within one year	$	—	$	—
After one year but within five years	37,852		39,985
After five years but within ten years	239,845		251,874
After ten years	2,172,568		2,237,198
	$	2,450,265	$	2,529,057


	Held-to-Maturity
	Amortized		Fair
(Dollars in thousands)	Cost		Value
March 31, 2021 (1)
Within one year	$	1,135	$	1,140
After one year but within five years	971		985
After five years but within ten years	41,075		42,558
After ten years	60,347		62,946
	$	103,528	$	107,629
December 31, 2020 (1)
Within one year	$	1,144	$	1,154
After one year but within five years	972		990
After five years but within ten years	35,967		37,317
After ten years	73,664		76,960
	$	111,747	$	116,421

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 5.0 years at March 31, 2021.

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 $1.4 billion and $1.3 billion were pledged as collateral for retail customer repurchase agreements, municipal deposits, and other obligations as of March 31, 2021 and December 31, 2020, respectively.

During the three months ended March 31, 2021, the Company sold $9.3 million of debt securities categorized as available-for-sale, resulting in $0.3 million of realized gains and no realized losses. During the three months ended March 31, 2020, the Company sold $30.6 million of debt securities categorized as available-for-sale resulting in $0.7 million of realized gains and no realized losses.

As of March 31, 2021 and December 31, 2020, the Company's debt securities portfolio had remaining unamortized premiums of $66.1 million and $60.4 million, respectively, and unaccreted discounts of $3.4 million and $2.6 million, respectively.

For debt securities in an unrealized loss position and an allowance has not been recorded, 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, 2021.


	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	$	288,625	$	15,223	$	—	$	—	$	288,625	$	15,223
FNMA MBS	1,241,199		34,243		4,330		3		1,245,529		34,246
FHLMC MBS	17,936		759		—		—		17,936		759
GNMA MBS	4,969		83		—		—		4,969		83
GSE agency notes	219,116		13,393		—		—		219,116		13,393
	$	1,771,845	$	63,701	$	4,330	$	3	$	1,776,175	$	63,704

Held-to-maturity debt securities:

Foreign bonds	$	500	$	1	$	—	$	—	$	500	$	1
	$	500	$	1	$	—	$	—	$	500	$	1

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 December 31, 2020.


	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	$	183,983	$	443	$	—	$	—	$	183,983	$	443
FNMA MBS	289,338		879		4,355		3		293,693		882
FHLMC MBS	5,191		105		—		—		5,191		105

GSE agency notes	101,016		649		—		—		101,016		649
	$	579,528	$	2,076	$	4,355	$	3	$	583,883	$	2,079

At March 31, 2021, available-for-sale debt securities for which the amortized cost basis exceeded fair value totaled $1.8 billion. Total unrealized losses on these securities were $63.7 million at March 31, 2021. 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, 2021.

At March 31, 2021 and December 31, 2020, held-to-maturity debt securities had an amortized cost basis of $103.5 million and $111.7 million, respectively. The held-to-maturity debt security portfolio primarily consists of highly rated municipal bonds. The Company monitors credit quality of its debt securities through credit ratings. The following table summarizes the amortized cost of debt securities held-to-maturity as of March 31, 2021, aggregated by credit quality indicator:


(Dollars in thousands)	State and political subdivisions		Foreign bonds
A+ rated or higher	$	102,901	$	501
Not rated	126		—
Ending balance	$	103,027	$	501

The following table summarizes the amortized cost of debt securities held-to-maturity as of December 31, 2020, aggregated by credit quality indicator:


(Dollars in thousands)	State and political subdivisions		Foreign bonds
A+ rated or higher	$	110,959	$	501
Not rated	287		—
Ending balance	$	111,246	$	501

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 foreign bond debt securities for the three months ended March 31, 2021 and 2020. 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, 2021 and 2020:


	Three months ended March 31,
(Dollars in thousands)	2021		2020
Allowance for credit losses:
Beginning balance	$	6	$	—
Impact of adoption ASC 326	—		8
Provision for credit losses	(1)		—

Charge-offs, net	—		—

Ending balance	$	5	$	8

Accrued interest receivable of $0.9 million and $1.1 million as of March 31, 2021 and December 31, 2020, 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, 2021 and December 31, 2020.