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

In adopting ASU 2017-12, the Company reassessed the classification of certain investments during the first quarter of 2019 and reclassified $573.6 million of securities from held to maturity to available for sale. The transfer occurred at fair value and had a related unrealized loss of $4.8 million recorded in other comprehensive income. During the fourth quarter of 2019, the Company reclassified the remaining $15.4 million of securities from held to maturity to available for sale in anticipation of the adoption of ASU 2016-13. The transfer occurred at fair value and had a related unrealized gain of $0.2 million recorded in other comprehensive income. At December 31, 2019, the Company did not hold any debt securities classified as held to maturity.

The amortized cost and fair value of debt securities as of December 31, 2019, 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 agencies and corporations (dollars in thousands).

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

(1)

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

Debt securities with carrying amounts of $704.7 million and $498.3 million on December 31, 2019 and 2018 respectively, were pledged as collateral for public deposits, securities sold under agreements to repurchase and for other purposes as required or permitted by law.

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, gross				12 months or more, gross				Total, gross
	Fair		Unrealized		Fair		Unrealized		Fair		Unrealized
December 31, 2019:	Value		Losses		Value		Losses		Value		Losses
Debt securities available for sale
U.S. Treasury securities	$	—	$	—	$	—	$	—	$	—	$	—
Obligations of U.S. government corporations and agencies		6,362		(48)		—		—		6,362		(48)
Obligations of states and political subdivisions⁽²⁾		4,981		(11)		1,548		—		6,529		(11)
Commercial mortgage-backed securities		33,322		(144)		2,044		(2)		35,366		(146)
Residential mortgage-backed securities		78,326		(245)		50,259		(379)		128,585		(624)
Corporate debt securities		—		—		—		—		—		—
Total temporarily impaired securities	$	122,991	$	(448)	$	53,851	$	(381)	$	176,842	$	(829)

(2)

Unrealized losses for greater than 12 months, gross, was less than one thousand dollars.


	Less than 12 months, gross				12 months or more, gross				Total, gross
	Fair		Unrealized		Fair		Unrealized		Fair		Unrealized
December 31, 2018:	Value		Losses		Value		Losses		Value		Losses
Debt securities available for sale
U.S. Treasury securities	$	995	$	(4)	$	24,343	$	(410)	$	25,338	$	(414)
Obligations of U.S. government corporations and agencies		749		(3)		50,744		(758)		51,493		(761)
Obligations of states and political subdivisions		49,893		(460)		77,651		(1,111)		127,544		(1,571)
Commercial mortgage-backed securities		—		—		1,942		(61)		1,942		(61)
Residential mortgage-backed securities		48,387		(496)		247,573		(6,647)		295,960		(7,143)
Corporate debt securities		90,713		(268)		15,083		(108)		105,796		(376)
Total temporarily impaired securities	$	190,737	$	(1,231)	$	417,336	$	(9,095)	$	608,073	$	(10,326)

Debt securities held to maturity
Obligations of states and political subdivisions	$	9,531	$	(33)	$	9,538	$	(54)	$	19,069	$	(87)
Commercial mortgage-backed securities		12,067		(212)		45,041		(791)		57,108		(1,003)
Residential mortgage-backed securities		77,071		(974)		245,128		(5,063)		322,199		(6,037)
Total temporarily impaired securities	$	98,669	$	(1,219)	$	299,707	$	(5,908)	$	398,376	$	(7,127)

Debt securities are periodically evaluated for OTTI. As of December 31, 2019, the Company’s debt security portfolio consisted of 1,178 securities. The total number of debt securities in the investment portfolio in an unrealized loss position as of December 31, 2019 was 55, and represented a loss of 0.47% of the aggregate amortized cost. The unrealized losses relate 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 Company does not consider these investments to be OTTI at December 31, 2019. As of December 31, 2019, the Company did not hold general obligation bonds of any single issuer, the aggregate of which exceeded 10% of the Company’s stockholders’ equity.