Investment Securities

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 periods ended March 31, 2018 and 2017, there were no investment securities deemed to be other-than-temporarily impaired.

Investment securities at March 31, 2018, were as follows:



Debt Securities Available-for-Sale (In thousands)	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses		Estimated Fair Value
U.S. Government sponsored entities' asset-backed securities	1,079,745		880		33,715		1,046,910
Total	$	1,079,745	$	880	$	33,715	$	1,046,910



Debt Securities Held-to-Maturity (In thousands)	Amortized Cost		Gross Unrecognized Holding Gains		Gross Unrecognized Holding Losses		Estimated Fair Value
U.S. Government sponsored entities' asset-backed securities	$	47,767	$	18	$	1,246	$	46,539
Obligations of states and political subdivisions	299,855		3,091		$	3,286	299,660
Total	$	347,622	$	3,109	$	4,532	$	346,199

Investment securities with unrealized/unrecognized losses at March 31, 2018, were as follows:



	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 Available-for-Sale
U.S. Government sponsored entities' asset-backed securities	662,276		17,518		$	294,713	16,197		$	956,989	33,715
Total	$	662,276	$	17,518	$	294,713	$	16,197	$	956,989	$	33,715
Debt Securities Held-to-Maturity
U.S. Government sponsored entities' asset-backed securities	$	45,521	$	1,246	$	—	$	—	$	45,521	$	1,246
Obligations of states and political subdivisions	115,143		$	1,534	49,748		1,752		$	164,891	3,286
Total	$	160,664	$	2,780	$	49,748	$	1,752	$	210,412	$	4,532

Investment securities at December 31, 2017, were as follows:



Debt Securities Available-for-Sale (In thousands)	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses		Estimated Fair Value
Obligations of U.S. Treasury and other U.S. Government sponsored entities	$	245,000	$	—	$	2,280	$	242,720
U.S. Government sponsored entities' asset-backed securities	852,645		4,645		8,129		849,161
Total	$	1,097,645	$	4,645	$	10,409	$	1,091,881



Debt Securities Held-to-Maturity (In thousands)	Amortized Cost		Gross Unrecognized Holding Gains		Gross Unrecognized Holding Losses		Estimated Fair Value
Obligations of states and political subdivisions	$	300,412	$	6,575	$	713	$	306,274
U.S. Government sponsored entities' asset-backed securities	56,785		758		38		57,505
Total	$	357,197	$	7,333	$	751	$	363,779

Investment securities with unrealized/unrecognized losses at December 31, 2017, were as follows:



	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 Available-for-Sale
Obligations of U.S. Treasury and other U.S. Government sponsored entities	$	24,931	$	70	$	217,789	$	2,210	$	242,720	$	2,280
U.S. Government sponsored entities' asset-backed securities	236,924		2,786		318,797		5,343		555,721		8,129
Total	$	261,855	$	2,856	$	536,586	$	7,553	$	798,441	$	10,409
Debt Securities Held-to-Maturity
Obligations of states and political subdivisions	$	26,644	$	194	$	45,498	$	519	$	72,142	$	713
U.S. Government sponsored entities' asset-backed securities	7,331		38		—		—		7,331		38
Total	$	33,975	$	232	$	45,498	$	519	$	79,473	$	751

Management does not believe any of the unrealized/unrecognized losses at March 31, 2018 or December 31, 2017 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 of 15-year residential mortgage-backed securities and collateralized mortgage obligations.

The amortized cost and estimated fair value of investments in debt securities at March 31, 2018, 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 Available-for-Sale (In thousands)	Amortized cost		Fair value		Tax equivalent yield
U.S. Government sponsored entities' asset-backed securities	$	1,079,745	$	1,046,910	2.28	%



Securities Held-to-Maturity (In thousands)	Amortized cost		Fair value		Tax equivalent yield⁽¹⁾
Obligations of state and political subdivisions:
Due five through ten years	$	2,446	$	2,420	2.97	%
Due over ten years	$	297,409	$	297,240	3.68	%
Total ⁽¹⁾	$	299,855	$	299,660	3.67	%

U.S. Government sponsored entities' asset-backed securities	$	47,767	$	46,539	2.83	%

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

The remaining average life of the entire investment portfolio is estimated to be 4.9 years.

During the three months ended March 31, 2018, Park sold certain AFS debt securities with a book value of $247.0 million at a loss of $2.6 million. Additionally, during the three months ended March 31, 2018, Park sold certain HTM debt securities with a book value of $7.4 million at a gain of $291,000. These HTM securities had been paid down by 96.3% of the principal outstanding at acquisition. There were no sales of investment securities during the three-month period ended March 31, 2017.

Investment securities having an amortized cost of $563 million and $557 million at March 31, 2018 and December 31, 2017, 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 ("FHLB") advance borrowings.