The amortized cost and fair value of investment debt securities AFS, with gross unrealized gains and losses, were as follows:


	As of March 31, 2021
(in thousands)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Allowance for Credit Loss related to Debt Securities		Fair Value

U.S. Government agencies and corporations	$	335	$	5	$	—	$	—	$	340
State and political subdivisions	672,120		9,510		7,707		—		673,923
Mortgage-backed securities	78,512		1,237		744		—		79,005
Collateralized mortgage obligations	717,112		4,546		5,677		—		715,981
Corporate debt securities	423,348		7,995		3,698		—		427,645
Total debt securities	$	1,891,427	$	23,293	$	17,826	$	—	$	1,896,894


	As of December 31, 2020
(in thousands)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Allowance for Credit Loss related to Debt Securities		Fair Value

U.S. Government agencies and corporations	$	355	$	6	$	—	$	—	$	361
State and political subdivisions	611,666		17,163		483		—		628,346
Mortgage-backed securities	92,261		1,758		1		—		94,018
Collateralized mortgage obligations	559,718		6,332		214		—		565,836
Corporate debt securities	360,103		9,333		616		—		368,820
Total debt securities	$	1,624,103	$	34,592	$	1,314	$	—	$	1,657,381

Investment securities with a fair value of $516.1 million and $434.7 million at March 31, 2021 and December 31, 2020, respectively, were pledged on public deposits, securities sold under agreements to repurchase and for other purposes, as required or permitted by law.

The following table presents debt securities AFS in an unrealized loss position for which an allowance for credit losses has not been recorded at March 31, 2021, aggregated by investment category and length of time in a continuous loss position:


		As of March 31, 2021
	Number of Securities	Less than 12 Months				12 Months or More				Total
Available for Sale		Fair Value		Unrealized Losses		Fair Value		Unrealized Losses		Fair Value		Unrealized Losses
(in thousands, except number of securities)


State and political subdivisions	145	$	274,770	$	7,413	$	9,468	$	294	$	284,238	$	7,707
Mortgage-backed securities	8	20,218		743		110		1		20,328		744
Collateralized mortgage obligations	22	392,272		5,677		—		—		392,272		5,677
Corporate debt securities	28	143,592		3,543		750		155		144,342		3,698
Total	203	$	830,852	$	17,376	$	10,328	$	450	$	841,180	$	17,826

As of March 31, 2021, 145 state and political subdivisions securities with total unrealized losses of $7.7 million were held by the Company. Management evaluated these securities through a process that included consideration of credit agency ratings and payment history. In addition, management may evaluate securities by considering the yield spread to treasury securities and the most recent financial information available. Based on this evaluation, management concluded that the decline in fair value was not attributable to credit losses.

As of March 31, 2021, 8 mortgage-backed securities and 22 collateralized mortgage obligations with unrealized losses totaling $6.4 million were held by the Company. Management evaluated the payment history of these securities. In addition, management considered the implied U.S. government guarantee of these agency securities and the level of credit enhancement for non-agency securities. Based on this evaluation, management concluded that the decline in fair value was not attributable to credit losses.

As of March 31, 2021, 28 corporate debt securities with total unrealized losses of $3.7 million were held by the Company. Management evaluated these securities by considering credit agency ratings and payment history. In addition, management may evaluate securities by considering the yield spread to treasury securities and the most recent financial information available. Based on this evaluation, management concluded that the decline in fair value was not attributable to credit losses.

Accrued interest receivable on available for sale debt securities, which is recorded within 'Other Assets,' totaled $7.8 million at March 31, 2021 and $7.3 million at December 31, 2020 and is excluded from the estimate of credit losses.

The following table presents debt securities AFS in an unrealized loss position for which an allowance for credit losses has not been recorded at December 31, 2020, aggregated by investment category and length of time in a continuous loss position:


		As of December 31, 2020
Available for Sale	Number of Securities	Less than 12 Months				12 Months or More				Total
		Fair Value		Unrealized Losses		Fair Value		Unrealized Losses		Fair Value		Unrealized Losses
(in thousands, except number of securities)


State and political subdivisions	27	$	31,489	$	157	$	4,065	$	326	$	35,554	$	483
Mortgage-backed securities	7	315		1		—		—		315		1
Collateralized mortgage obligations	8	133,032		214		—		—		133,032		214
Corporate debt securities	15	35,995		523		3,311		93		39,306		616
Total	57	$	200,831	$	895	$	7,376	$	419	$	208,207	$	1,314

Proceeds and gross realized gains and losses on debt securities available for sale for the three months ended March 31, 2021 and 2020 were as follows:


	Three Months Ended
(in thousands)	March 31, 2021		March 31, 2020
Proceeds from sales of debt securities available for sale	$	—	$	22,140

Gross realized gains from sales of debt securities available for sale	—		155
Gross realized losses from sales of debt securities available for sale	—		(113)
Net realized gain from sales of debt securities available for sale	$	—	$	42

The contractual maturity distribution of investment debt securities at March 31, 2021, is shown below. Expected maturities of MBS and CMO may differ from contractual maturities because the mortgages underlying the securities may be called or prepaid without any penalties. Therefore, these securities are not included in the maturity categories in the following summary.


	Available For Sale
(in thousands)	Amortized Cost		Fair Value
Due in one year or less	$	35,751	$	36,169
Due after one year through five years	304,392		310,792
Due after five years through ten years	418,530		416,063
Due after ten years	337,130		338,884
	$	1,095,803	$	1,101,908
Mortgage-backed securities	78,512		79,005
Collateralized mortgage obligations	717,112		715,981
Total	$	1,891,427	$	1,896,894