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 transferred $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.

The 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 agencies and corporations (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 (losses) gains on sales of securities reported on the unaudited Consolidated Statements of Income includes sale of equity securities, excluded in this table.

Debt securities with carrying amounts of $768.7 million and $498.3 million on September 30, 2019 and December 31, 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):



	For less than				For greater
	12 months, gross				than 12 months, gross				Total, gross
	Fair		Unrealized		Fair		Unrealized		Fair		Unrealized
September 30, 2019:	Value		Losses		Value		Losses		Value		Losses
Debt securities available for sale
U.S. Treasury securities	$	—	$	—	$	9,982	$	(6)	$	9,982	$	(6)
Obligations of U.S. government corporations and agencies		6,938		(45)		6,481		(8)		13,419		(53)
Obligations of states and political subdivisions		4,949		(11)		9,129		(5)		14,078		(16)
Commercial mortgage-backed securities		3,913		(11)		9,702		(7)		13,615		(18)
Residential mortgage-backed securities		43,372		(47)		57,319		(560)		100,691		(607)
Corporate debt securities		485		(2)		5,595		(3)		6,080		(5)
Total temporarily impaired securities	$	59,657	$	(116)	$	98,208	$	(589)	$	157,865	$	(705)



	For less than				For greater
	12 months, gross				than 12 months, 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 other-than-temporary impairment (“OTTI”). As of September 30, 2019, the Company’s debt security portfolio consisted of 1,208 securities. The total number of debt securities in the investment portfolio in an unrealized loss position as of September 30, 2019 was 74 and represented an unrealized loss of 0.44% of the aggregate fair value. 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 September 30, 2019. As of September 30, 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.