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


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

U.S. Government agencies and corporations	$	421	$	10	$	—	$	—	$	431
State and political subdivisions	283,087		4,499		1,209		—		286,377
Mortgage-backed securities	41,191		1,412		2		—		42,601
Collateralized mortgage obligations	301,287		6,804		1,231		—		306,860
Corporate debt securities	245,674		3,343		3,427		—		245,590
Total debt securities	$	871,660	$	16,068	$	5,869	$	—	$	881,859


	As of December 31, 2019
(in thousands)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value

U.S. Government agencies and corporations	$	439	$	2	$	—	$	441
State and political subdivisions	253,750		3,803		348		257,205
Mortgage-backed securities	43,009		536		15		43,530
Collateralized mortgage obligations	293,911		1,000		1,965		292,946
Corporate debt securities	188,952		3,018		115		191,855
Total debt securities	$	780,061	$	8,359	$	2,443	$	785,977

Investment securities with a carrying value of $334.7 million and $264.8 million at March 31, 2020 and December 31, 2019, 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, 2020, aggregated by investment category and length of time in a continuous loss position:


		As of March 31, 2020
	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	77	$	33,845		$	1,209		$	—		$	—		$	33,845		$	1,209
Mortgage-backed securities	9	404			2			17			—			421			2
Collateralized mortgage obligations	8	62,873			1,231			—			—			62,873			1,231
Corporate debt securities	28	105,435			3,342			1,316			85			106,751			3,427
Total	122	$	202,557		$	5,784		$	1,333		$	85		$	203,890		$	5,869

As of March 31, 2020, 77 state and political subdivisions securities with total unrealized losses of $1.2 million were held by the Company. Management evaluated these securities by considering the yield spread to treasury securities, credit agency ratings, and payment history. In addition, management evaluated the most recent financial information available for certain of these securities. Based on this evaluation, management concluded that the decline in fair value was not attributable to credit losses.

As of March 31, 2020, 9 mortgage-backed securities and 8 collateralized mortgage obligations with unrealized losses 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, 2020, 28 corporate debt securities with total unrealized losses of $3.4 million were held by the Company. Management evaluated these securities by considering credit agency ratings and payment history. In addition, management evaluated the most recent financial information available for certain of these securities. 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 $4.6 million at March 31, 2020 and is excluded from the estimate of credit losses.

The following table presents information pertaining to debt securities with gross unrealized losses as of December 31, 2019, aggregated by investment category and length of time that individual securities have been in a continuous loss position:


		As of December 31, 2019
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	47	$	27,161	$	322	$	2,112	$	26	$	29,273	$	348
Mortgage-backed securities	7	963		12		1,365		3		2,328		15
Collateralized mortgage obligations	33	103,395		719		65,604		1,246		168,999		1,965
Corporate debt securities	7	7,012		14		8,788		101		15,800		115
Total	94	$	138,531	$	1,067	$	77,869	$	1,376	$	216,400	$	2,443

Proceeds and gross realized gains and losses on debt securities available for sale for the three months ended March 31, 2020 and 2019 were as follows:


	Three Months Ended
(in thousands)	March 31, 2020		March 31, 2019
Proceeds from sales of debt securities available for sale	$	22,140	$	7,280

Gross realized gains from sales of debt securities available for sale	155		26
Gross realized losses from sales of debt securities available for sale	(113)		(9)
Net realized gain from sales of debt securities available for sale	$	42	$	17

The contractual maturity distribution of investment debt securities at March 31, 2020, 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	$	42,982	$	43,030
Due after one year through five years	185,876		186,530
Due after five years through ten years	225,762		227,898
Due after ten years	74,562		74,940
	$	529,182	$	532,398
Mortgage-backed securities	41,191		42,601
Collateralized mortgage obligations	301,287		306,860
Total	$	871,660	$	881,859