All of the mortgage-backed securities (“MBS”) listed below are residential Fannie Mae, Freddie Mac and Ginnie Mae MBSs. The amortized cost, fair value and allowance for credit losses of available for sale debt securities and the related gross unrealized gains and losses recognized in accumulated other comprehensive income (loss) were as follows:

	March 31, 2020
		Gross	Gross	Allowance
	Amortized	unrealized	unrealized	for credit	Fair
	cost	gains	losses	losses	value
Corporate debt securities	$5,504	$253	$ —	$ —	$5,757
Obligations of U.S. government sponsored entities and agencies	147,339	620	—	—	147,959
Mortgage-backed securities	1,711,738	76,108	—	—	1,787,846
U.S. treasuries	100,001	—	4	—	99,997
Municipal securities	90,162	6,721	—	—	96,883
Total available for sale debt securities	$2,054,744	$83,702	$4	$ —	$2,138,442

	December 31, 2019
		Gross	Gross
	Amortized	Unrealized	Unrealized	Fair
	Cost	Gains	Losses	Value
Corporate debt securities	$5,504	$153	$ —	$5,657
Obligations of U.S. government sponsored entities and agencies	20,000	—	70	19,930
Mortgage-backed securities	1,742,221	26,403	1,382	1,767,242
Municipal securities	88,301	5,594	—	93,895
Total available for sale debt securities	$1,856,026	$32,150	$1,452	$1,886,724

The cost of securities sold is determined using the specific identification method. No available for sale debt securities were sold during the three-month ended March 31, 2020. Sales of available for sale debt securities for the three-month ended March 31, 2019 were as follows:

The fair value of available for sale debt securities at March 31, 2020 by contractual maturity were as follows. Securities not due at a single maturity date, primarily mortgage-backed securities, are shown separately.

	Fair	Amortized
Investment securities available for sale:	Value	Cost
Due one year or less	$102,204	$102,191
Due after one year through five years	131,668	131,295
Due after five years through ten years	45,763	44,017
Due after ten years through thirty years	70,961	65,503
Mortgage-backed securities	1,787,846	1,711,738
Total available for sale debt securities	$2,138,442	$2,054,744

Available for sale debt securities pledged at March 31, 2020 and December 31, 2019 had a carrying amount (estimated fair value) of $1,403,169 and $1,195,664, respectively. These securities were pledged primarily to increase borrowing capacity at the FHLB, secure public deposits and repurchase agreements. In addition, the amounts at March 31, 2020 and December 31, 2019 include $605,100 and $361,127 of securities pledged to the third party dealers and clearinghouse exchanges for the Company’s cash flow hedge interest rate swaps, respectively.

At March 31, 2020 and December 31, 2019, there were no holdings of securities of any one issuer, other than mortgage-backed securities issued by U.S. Government sponsored entities, in an amount greater than 10% of stockholders’ equity.

The following tables show the Company’s available for sale debt investments’ gross unrealized losses and fair value, aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position, at March 31, 2020 and December 31, 2019.

	March 31, 2020
	Less than 12 months		12 months or more		Total
	Fair	Unrealized	Fair	Unrealized	Fair	Unrealized
	value	losses	value	losses	value	losses
Mortgage-backed securities	$100	$ —	$ —	$ —	$100	$ —
U.S. treasuries	99,997	4	—	—	99,997	4
Total temporarily impaired available for sale debt securities	$100,097	$4	$ —	$ —	$100,097	$4

	December 31, 2019
	Less than 12 months		12 months or more		Total
	Fair	Unrealized	Fair	Unrealized	Fair	Unrealized
	Value	Losses	Value	Losses	Value	Losses
Obligations of U.S. government sponsored entities and agencies	$19,930	$70	$ —	$ —	$19,930	$70
Mortgage-backed securities	125,249	388	117,903	994	243,152	1,382
Total temporarily impaired available for sale debt securities	$145,179	$458	$117,903	$994	$263,082	$1,452

Mortgage-backed securities: At March 31, 2020, 100% of the mortgage-backed securities held by the Company were issued by U.S. government-sponsored entities and agencies, primarily Fannie Mae, Freddie Mac, and Ginnie Mae, institutions which the government has affirmed its commitment to support. As of March 31, 2020, none of our mortgage-backed securities were in a loss position.

U.S. treasuries: U.S. treasuries have almost no credit risk since they are backed by the full faith and credit of the U.S. government. Because the decline in fair value is attributable to changes in interest rates and inflation, and not credit quality, and because the Company does not have the intent to sell these securities and it is likely that it will not be required to sell the securities before their anticipated recovery, the Company does not consider these securities to be other-than-temporarily impaired at March 31, 2020.

Mortgage-Backed Securities: Most of the held to maturity securities investment portfolio is invested in U.S. Government Agency MBS. Given that the principal and interest payments on these securities are guaranteed by a U.S. Government Agency, there is minimal risk. Default from mortgage-backed securities would be present in macroeconomic events such as a recession which would result in insufficient cash flow to the servicer to continue regular payments.

Municipal Securities: Defaults on municipal bonds are not common, but they are not without risk. Risks stem from the potential for financial mismanagement of the municipal entity or a significant deterioration in the tax or revenue base of the municipal entity.

The following reflects the amortized cost, fair value and allowance for credit losses and the related gross unrecognized gains and losses of held to maturity securities as of March 31, 2020 and December 31, 2019.

	March 31, 2020
		Gross	Gross		Allowance
	Amortized	unrecognized	unrecognized	Fair	for credit
	cost	gains	losses	value	losses
Mortgage-backed securities	$67,841	$2,317	$ —	$70,158	$ —
Municipal securities	128,117	7,183	—	135,300	(10)
Total held to maturity debt securities	$195,958	$9,500	$ —	$205,458	$(10)

	December 31, 2019
		Gross	Gross
	Amortized	unrecognized	unrecognized	Fair
	cost	gains	losses	value
Mortgage-backed securities	$71,560	$456	$29	$71,987
Municipal securities	131,343	5,522	—	136,865
Total held to maturity debt securities	$202,903	$5,978	$29	$208,852

Held to maturity securities pledged at March 31, 2020 and December 31, 2019 had a carrying amount of $130,281 and $100,582 respectively. These securities were pledged primarily to secure public deposits and repurchase agreements.

At March 31, 2020, there were no holdings of held to maturity securities of any one issuer, other than the U.S. Government and its agencies, in an amount greater than 10% of stockholders’ equity.

The fair value and amortized cost of held to maturity securities at March 31, 2020 by contractual maturity were as follows. Mortgage-backed securities are not due at a single maturity date and are shown separately.

	Fair	Amortized
Held to maturity debt securities	value	cost
Due after five years through ten years	$2,193	$2,138
Due after ten years through thirty years	133,107	125,979
Mortgage-backed securities	70,158	67,841
Total held to maturity debt securities	$205,458	$195,958

As of March 31, 2020, there were no held to maturity debt securities in an unrecognized loss position. The following table shows the Company’s held to maturity debt investments’ gross unrecognized losses and fair value, aggregated by investment category and length of time the individual securities have been in a continuous unrecognized loss position, at December 31, 2019.

	December 31, 2019
	Less than 12 months		12 months or more		Total
	Fair	Unrecognized	Fair	Unrecognized	Fair	Unrecognized
	Value	Losses	Value	Losses	Value	Losses
Mortgage-backed securities	$ —	$ —	$8,445	$29	$8,445	$29
Total temporarily impaired held to maturity debt securities	$ —	$ —	$8,445	$29	$8,445	$29

The following table shows a roll-forward for the three-month ended March 31, 2020 for the of the allowance for credit losses on held to maturity debt investments:

			Municipal
Held to maturity debt securities			securities
Allowance for credit losses:
Beginning balance, January 1, 2020			$ —
Impact of adopting ASC 326			10
Provision for credit loss expense			—
Allowance on purchased financial assets with credit deterioration			—
Securities charged off			—
Recoveries			—
Total ending allowance balance			$10

The Company adopted CECL effective January 1, 2020 and recorded allowance for credit losses of $10 for held to maturity debt securities. The Company did not record any provision for credit loss on held to maturity debt securities for the three-month ended March 31, 2020.

Pursuant to ASC 326, the Company must also determine if the decline in fair value of investment securities, relative to its amortized cost, is due to credit-related factors. With respect to U.S. Government agency securities, the Bank has determined that a decline in fair value is not due to credit-related factors. In addition, no held to maturity debt securities were past due or on non-accrual as of March 31, 2020. The Company monitors the credit quality of debt securities held to maturity through the use of credit rating and other factors specific to an individual security in assessing whether or not the decline in fair value of municipal and corporate securities, relative to their amortized cost, is due to credit-related factors. The Company uses the following triggers to prompt further investigation of securities when the fair value is less than amortized costs: the security has been downgraded and fallen below an A credit rating, and the security’s unrealized loss exceeds 20% of its book value. The Company monitors credit ratings quarterly and annually performs an internal credit review of its municipal securities.

The following table summarizes the amortized costs of held to maturity debt securities at March 31, 2020, aggregated by credit quality indicator:

	Mortgage-backed	Municipal
	securities	securities
At March 31, 2020
AAA/AA/A	$ —	$126,737
Not rated (1)	67,841	1,380
Total	$67,841	$128,117