The amortized cost and fair value of investment securities are shown in the following tables. Management performs a quarterly evaluation of investment securities for any other-than-temporary impairment. For the three-month and nine-month periods ended September 30, 2019 and 2018, there were no investment securities deemed to be other-than-temporarily impaired.



Debt Securities AFS (In thousands)	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses		Estimated Fair Value
Obligations of states and political subdivisions	$	303,522	$	18,383	$	—	$	321,905
U.S. Government sponsored entities' asset-backed securities	934,468		10,373		6,460		938,381
Total	$	1,237,990	$	28,756	$	6,460	$	1,260,286



	Unrealized loss position for less than 12 months				Unrealized loss position for 12 months or longer				Total
(In thousands)	Fair value		Unrealized losses		Fair value		Unrealized losses		Fair value		Unrealized losses
Debt Securities AFS
U.S. Government sponsored entities' asset-backed securities	$	251,414	$	1,095	$	181,807	$	5,365	$	433,221	$	6,460
Total	$	251,414	$	1,095	$	181,807	$	5,365	$	433,221	$	6,460



Debt Securities AFS (In thousands)	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses		Estimated Fair Value
U.S. Government sponsored entities' asset-backed securities	$	1,028,883	$	453	$	25,915	$	1,003,421
Total	$	1,028,883	$	453	$	25,915	$	1,003,421



Debt Securities HTM (In thousands)	Amortized Cost		Gross Unrecognized Holding Gains		Gross Unrecognized Holding Losses		Estimated Fair Value
Obligations of states and political subdivisions	$	305,278	$	3,202	$	2,672	$	305,808
U.S. Government sponsored entities' asset-backed securities	46,530		87		1,003		45,614
Total	$	351,808	$	3,289	$	3,675	$	351,422



	Unrealized/unrecognized loss position for less than 12 months				Unrealized/unrecognized loss position for 12 months or longer				Total
(In thousands)	Fair value		Unrealized/unrecognized losses		Fair value		Unrealized/unrecognized losses		Fair value		Unrealized/unrecognized losses
Debt Securities AFS
U.S. Government sponsored entities' asset-backed securities	$	506,280	$	5,998	$	449,569	$	19,917	$	955,849	$	25,915
Total	$	506,280	$	5,998	$	449,569	$	19,917	$	955,849	$	25,915
Debt Securities HTM
Obligations of states and political subdivisions	$	91,960	$	1,095	$	70,723	$	1,577	$	162,683	$	2,672
U.S. Government sponsored entities' asset-backed securities	32,656		838		6,931		165		39,587		1,003
Total	$	124,616	$	1,933	$	77,654	$	1,742	$	202,270	$	3,675

Management does not believe any of the unrealized/unrecognized losses at September 30, 2019 or December 31, 2018 represented other-than-temporary impairment. Should the impairment of any of these securities become other-than-temporary, the cost basis of the investment will be reduced and the resulting loss recognized within net income in the period the other-than-temporary impairment is identified.

Park’s U.S. Government sponsored entities' asset-backed securities consist primarily of 15-year residential mortgage-backed securities and collateralized mortgage obligations.

On September 1, 2019, Park adopted the portion of ASU 2019-04 which allowed for a one time reclassification of securities from HTM to AFS. On this date, Park transferred HTM securities with a fair value of $373.9 million to AFS classification. The transfer occurred at fair value and had a related unrealized gain, net of taxes, of $19.1 million recorded in other comprehensive income.

The amortized cost and estimated fair value of investments in debt securities at September 30, 2019, are shown in the following table by contractual maturity, except for asset-backed securities, which are shown as a single total, due to the unpredictability of the timing of principal repayments.



Securities AFS (In thousands)	Amortized cost		Fair value		Tax equivalent yield ⁽¹⁾

U.S. Government sponsored entities' asset-backed securities	$	934,468	$	938,381	2.37	%

Obligations of state and political subdivisions:
Due five through ten years	$	3,998	$	4,140	3.04	%
Due over ten years	299,524		317,765		3.69	%
Total ⁽¹⁾	$	303,522	$	321,905	3.68	%

(1) The tax equivalent yield for certain obligations of state and political subdivisions includes the effects of a taxable equivalent adjustment using a 21% federal corporate income tax rate.

During the three-month period ended September 30, 2019, Park sold certain AFS debt securities with a book value of $10.7 million at a gross loss of $67,000, with a tax benefit of $14,000, and sold certain AFS debt securities with a book value of $23.8 million at a gross gain of $253,000, with tax expense of $53,000. During the nine-month period ended September 30, 2019, Park sold certain AFS debt securities with a book value of $62.4 million at a gross loss of $692,000, with a tax benefit of $145,000, and sold certain AFS debt securities with a book value of $29.1 million at a gross gain of $271,000, with tax expense of $57,000.

There were no sales of investment securities during the three-month period ended September 30, 2018. During the nine-month period ended September 30, 2018, Park sold certain AFS debt securities with a book value of $245.0 million at a gross loss of $2.6 million, with a tax benefit of $551,000, and sold certain AFS debt securities with a book value of $2.0 million at a gross gain of $60,000, with tax expense of $13,000. Additionally, during the nine-month period ended September 30, 2018, Park sold certain HTM debt securities with a book value of $7.4 million at a gross gain of $291,000, with tax expense of $61,000. These HTM securities had been paid down by 96.3% of the principal outstanding at acquisition.

Investment securities having a book value of $574 million and $634 million at September 30, 2019 and December 31, 2018, respectively, were pledged to collateralize government and trust department deposits in accordance with federal and state requirements, to secure repurchase agreements sold and as collateral for Federal Home Loan Bank advance borrowings.