Investment securities at June 30, 2021 and December 31, 2020, were as follows:


Debt securities AFS (In thousands)	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses		Fair Value
June 30, 2021:
Obligations of states and political subdivisions	$	278,104	$	23,726	$	—	$	301,830
U.S. Government sponsored entities' asset-backed securities	856,255		20,132		3,479		872,908
Collateralized loan obligations	227,214		7		23		227,198
Corporate debt securities	2,250		82		—		2,332
Total	$	1,363,823	$	43,947	$	3,502	$	1,404,268


Debt securities AFS (In thousands)	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses		Fair Value
December 31, 2020:
Obligations of states and political subdivisions	$	279,245	$	25,973	$	—	$	305,218
U.S. Government sponsored entities' asset-backed securities	726,589		26,248		728		752,109
Corporate debt securities	2,000		14		—		2,014
Total	$	1,007,834	$	52,235	$	728	$	1,059,341

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


	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	$	247,507	$	3,479	$	—	$	—	$	247,507	$	3,479
Collateralized loan obligations	55,123		23		—		—		55,123		23
Total	$	302,630	$	3,502	$	—	$	—	$	302,630	$	3,502


	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	$	86,393	$	695	$	4,727	$	33	$	91,120	$	728
Total	$	86,393	$	695	$	4,727	$	33	$	91,120	$	728

Unrealized losses on U.S. Government sponsored entities' asset-based securities and collateralized loan obligations have not been recognized into income as they represent negative adjustments to fair value relative to the rate of interest paid on the securities and not losses related to the creditworthiness of the issuer. Management does not intend to sell, and it is not more likely than not that management would be required to sell, the securities prior to their anticipated recovery. Management believes the value will recover as the securities approach maturity or market rates change.

There was no allowance for credit losses recorded for AFS debt securities at June 30, 2021. Additionally, for the three-months and six-months ended June 30, 2021 and 2020, there were no credit-related investment impairment losses recognized.

The amortized cost and estimated fair value of investments in debt securities at June 30, 2021, are shown in the following table by contractual maturity, except for asset-backed securities and collateral loan obligations, which are shown as a single total, due to the unpredictability of the timing of principal repayments.


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

U.S. Government sponsored entities' asset-backed securities	$	856,255	$	872,908	1.82	%

Collateralized loan obligations	$	227,214	$	227,198	1.40	%

Corporate debt securities
Due five through ten years	$	2,250	$	2,332	3.96	%

Obligations of state and political subdivisions:
Due five through ten years	$	101,573	$	110,366	3.77	%
Due over ten years	176,531		191,464		3.66	%
Total (1)	$	278,104	$	301,830	3.70	%

(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.

There were no sales of AFS debt securities during the three-month or six-month periods ended June 30, 2021. During both the three-month and the six-month periods ended June 30, 2020, Park sold certain AFS debt securities with a book value of $55.5 million at a gross gain of $3.3 million.

Investment securities having an amortized cost of $710 million and $691 million at June 30, 2021 and December 31, 2020, 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 FHLB advance borrowings.