The table below provides the amortized cost, unrealized gains and losses, and fair values of debt securities summarized by major category (dollars in thousands):

Amortized cost and fair value of debt securities by contractual maturity or pre-refunded date are shown below. Mortgages underlying mortgage-backed securities may be called or prepaid; therefore, actual maturities could differ from the contractual maturities. All mortgage-backed securities were issued by U.S. government corporations and agencies (dollars in thousands):

Realized gains and losses related to sales and calls of debt securities available for sale are summarized as follows (dollars in thousands):

(1)

Net gains (losses) on sales of securities reported on the unaudited Consolidated Statements of Income includes sales of equity securities, excluded in this table.

Debt securities with carrying amounts of $562.6 million on March 31, 2021, and $628.0 million on December 31, 2020, were pledged as collateral for public deposits, securities sold under agreements to repurchase, and for other purposes as required.

The following information pertains to debt securities with gross unrealized losses, aggregated by investment category and the length of time that individual securities have been in a continuous loss position (dollars in thousands):


	Less than 12 months				12 months or more				Total
	Fair		Unrealized		Fair		Unrealized		Fair		Unrealized
March 31, 2021:	Value		Losses		Value		Losses		Value		Losses
Debt securities available for sale
Obligations of U.S. government corporations and agencies	$	—	$	—	$	4,695	$	(45)	$	4,695	$	(45)
Obligations of states and political subdivisions		58,555		(1,542)		—		—		58,555		(1,542)
Commercial mortgage-backed securities		314,109		(6,786)		—		—		314,109		(6,786)
Residential mortgage-backed securities		1,019,462		(18,325)		352		(4)		1,019,814		(18,329)
Corporate debt securities		95,164		(1,427)		—		—		95,164		(1,427)
Total temporarily impaired securities	$	1,487,290	$	(28,080)	$	5,047	$	(49)	$	1,492,337	$	(28,129)


	Less than 12 months				12 months or more				Total
	Fair		Unrealized		Fair		Unrealized		Fair		Unrealized
December 31, 2020:	Value		Losses		Value		Losses		Value		Losses
Debt securities available for sale
Obligations of U.S. government corporations and agencies	$	—	$	—	$	4,957	$	(49)	$	4,957	$	(49)
Obligations of states and political subdivisions		762		(8)		—		—		762		(8)
Commercial mortgage-backed securities		129,655		(312)		—		—		129,655		(312)
Residential mortgage-backed securities		89,997		(300)		139		(3)		90,136		(303)
Corporate debt securities		1,499		(1)		—		—		1,499		(1)
Total temporarily impaired securities	$	221,913	$	(621)	$	5,096	$	(52)	$	227,009	$	(673)

Debt securities available for sale are not within the scope of CECL, however, the accounting for credit losses on these securities is affected by ASC 326-30. As of March 31, 2021, the Company’s debt security portfolio consisted of 1,108 securities, compared to 1,114 securities at December 31, 2020. The total number of debt securities in the investment portfolio in an unrealized loss position as of March 31, 2021, was 192 and represented an unrealized loss of 1.88% of the aggregate fair value. The total number of debt securities in the investment portfolio in an unrealized loss position as of December 31, 2020, was 23 and represented an unrealized loss of 0.30% of the aggregate fair value. Unrealized losses related to changes in market interest rates and market conditions that do not represent credit-related impairments. Furthermore, the Company does not intend to sell such securities and it is more likely than not that the Company will recover the amortized cost prior to being required to sell the debt securities. Full collection of the amounts due according to the contractual terms of the debt securities is expected; therefore, the impairment related to noncredit factors is recognized in accumulated other comprehensive income (loss), net of applicable taxes. As of March 31, 2021, the Company did not hold general obligation bonds of any single issuer, the aggregate of which exceeded 10% of the Company’s stockholders’ equity.