Held-to-maturity securities, which include any security for which the Company has the positive intent and ability to hold until maturity, are carried at historical cost adjusted for amortization of premiums and accretion of discounts. Premiums and discounts are amortized and accreted, respectively, to interest income using the constant yield method over the period to maturity.

Available-for-sale securities, which include any security for which the Company has no immediate plan to sell but which may be sold in the future, are carried at fair value. Realized gains and losses, based on specifically identified amortized cost of the individual security, are included in other income. Unrealized gains and losses are recorded, net of related income tax effects, in stockholders’ equity, further discussed below. Premiums and discounts are amortized and accreted, respectively, to interest income using the constant yield method over the period to maturity.


(In thousands)	Amortized Cost		Allowance for Credit Losses		Net Carrying Amount		Gross Unrealized Gains		Gross Unrealized (Losses)		Estimated Fair Value
Held-to-maturity

December 31, 2020
Mortgage-backed securities	$	22,354	$	—	$	22,354	$	683	$	—	$	23,037
State and political subdivisions	312,416		(2,307)		310,109		8,148		(30)		318,227
Other securities	1,176		(608)		568		93		—		661
Total HTM	$	335,946	$	(2,915)	$	333,031	$	8,924	$	(30)	$	341,925

December 31, 2019
Mortgage-backed securities	$	10,796	$	—	$	10,796	$	71	$	(59)	$	10,808
State and political subdivisions	27,082		—		27,082		849		—		27,931
Other securities	3,049		—		3,049		67		—		3,116
Total HTM	$	40,927	$	—	$	40,927	$	987	$	(59)	$	41,855


(In thousands)	Amortized Cost		Allowance for Credit Losses		Gross Unrealized Gains		Gross Unrealized (Losses)		Estimated Fair Value
Available-for-sale

December 31, 2020
U.S. Government agencies	$	477,693	$	—	$	844	$	(1,300)	$	477,237
Mortgage-backed securities	1,374,769		—		21,261		(1,094)		1,394,936
State and political subdivisions	1,416,136		(217)		55,111		(307)		1,470,723
Other securities	128,445		(95)		2,447		(95)		130,702
Total AFS	$	3,397,043	$	(312)	$	79,663	$	(2,796)	$	3,473,598

December 31, 2019
U.S. Treasury	$	449,729	$	—	$	112	$	(112)	$	449,729
U.S. Government agencies	194,207		—		1,313		(1,271)		194,249
Mortgage-backed securities	1,738,584		—		8,510		(4,149)		1,742,945
State and political subdivisions	860,539		—		20,983		(998)		880,524
Other securities	20,092		—		822		(18)		20,896
Total AFS	$	3,263,151	$	—	$	31,740	$	(6,548)	$	3,288,343

Accrued interest receivable on HTM and AFS securities at December 31, 2020 was $1.0 million and $17.1 million, respectively, and is included in interest receivable on the consolidated balance sheets. The Company has made the election to exclude all accrued interest receivable from securities from the estimate of credit losses.

The following table summarizes the Company’s AFS investments in an unrealized loss position for which an allowance for credit loss has not been recorded as of December 31, 2020, aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position:


	Less Than 12 Months				12 Months or More				Total
(In thousands)	Estimated Fair Value		Gross Unrealized Losses		Estimated Fair Value		Gross Unrealized Losses		Estimated Fair Value		Gross Unrealized Losses
Available-for-sale
U.S. Government agencies	$	228,241	$	(432)	$	52,853	$	(868)	$	281,094	$	(1,300)
Mortgage-backed securities	264,843		(1,082)		3,703		(12)		268,546		(1,094)
State and political subdivisions	9,302		(88)		339		(2)		9,641		(90)
Total AFS	$	502,386	$	(1,602)	$	56,895	$	(882)	$	559,281	$	(2,484)

As of December 31, 2020, the Company’s investment portfolio included $3.5 billion of AFS securities, of which $559.3 million, or 16.1%, were in an unrealized loss position that are not deemed to have credit losses. A portion of the unrealized losses were related to the Company’s mortgage-backed securities, which are issued and guaranteed by U.S. government-sponsored entities and agencies, and the Company’s state and political securities, specifically investments in insured fixed rate municipal bonds for which the issuers continue to make timely principal and interest payments under the contractual terms of the securities.

Furthermore, the decline in fair value for each of the above AFS securities is attributable to the rates for those investments yielding less than current market rates. Management does not believe any of the securities are impaired due to reasons of credit quality. Management believes the declines in fair value for the securities are temporary. Management does not have the intent to sell the securities, and management believes it is more likely than not the Company will not have to sell the securities before recovery of their amortized cost basis.

All of the mortgage-backed securities held by the Company are issued by U.S. government-sponsored entities and agencies. These securities are either explicitly or implicitly guaranteed by the U.S. government, are highly rated by major rating agencies and have a long history of no credit losses. Accordingly, no allowance for credit losses has been recorded for these securities.

Regarding securities issued by state and political subdivisions and other HTM securities, management considers (i) issuer bond ratings, (ii) historical loss rates for given bond ratings, (iii) whether issuers continue to make timely principal and interest payments under the contractual terms of the securities, (iv) internal forecasts, (v) whether or not such securities provide insurance or other credit enhancement or are pre-refunded by the issuers.

The following table details activity in the allowance for credit losses by investment security type for the year ended December 31, 2020 on the Company’s HTM and AFS securities held.


(In thousands)	State and Political Subdivisions		Other Securities		Total
Held-to-maturity
Beginning balance, January 1, 2020	$	—	$	—	$	—
Impact of ASU 2016-13 adoption	58		311		369
Provision for credit loss expense	2,249		297		2,546
Ending balance, December 31, 2020	$	2,307	$	608	$	2,915

Available-for-sale
Beginning balance, January 1, 2020	$	—	$	—	$	—
Impact of ASU 2016-13 adoption	373		—		373
Credit losses on securities not previously recorded	199		113		312
Reduction due to sales	(244)		—		(244)
Net decrease in allowance on previously impaired securities	(111)		(18)		(129)
Ending balance, December 31, 2020	$	217	$	95	$	312


	State and Political Subdivisions
(In thousands)	Not Guaranteed or Pre-Refunded		Other Credit Enhancement or Insurance		Pre-Refunded		Total		Other Securities
Aaa/AAA	$	52,058	$	5,828	$	—	$	57,886	$	—
Aa/AA	181,481		49,788		—		231,269		—
A	19,838		1,022		—		20,860		—
Not Rated	1,998		403		—		2,401		1,176
Total	$	255,375	$	57,041	$	—	$	312,416	$	1,176

Historical loss rates associated with securities having similar grades as those in the Company’s portfolio have generally not been significant. Pre-refunded securities, if any, have been defeased by the issuer and are fully secured by cash and/or U.S. Treasury securities held in escrow for payment to holders when the underlying call dates of the securities are reached. Securities with other credit enhancement or insurance continue to make timely principal and interest payments under the contractual terms of the securities. Accordingly, no allowance for credit losses has been recorded for these securities as there is no current expectation of credit losses related to these securities.


(In thousands)	2020		2019		2018
Taxable:
Held-to-maturity	$	985	$	1,207	$	2,157
Available-for-sale	34,054		42,412		37,068

Non-taxable:
Held-to-maturity	919		1,412		7,424
Available-for-sale	28,575		18,345		6,811
Total	$	64,533	$	63,376	$	53,460

The amortized cost and estimated fair value by maturity of securities are shown in the following table. Securities are classified according to their contractual maturities without consideration of principal amortization, potential prepayments or call options. Accordingly, actual maturities may differ from contractual maturities.


	Held-to-Maturity				Available-for-Sale
(In thousands)	Amortized Cost		Fair Value		Amortized Cost		Fair Value
One year or less	$	5,821	$	5,895	$	14,017	$	14,089
After one through five years	10,708		11,199		30,634		31,071
After five through ten years	5,227		2,937		229,042		232,003
After ten years	291,836		298,857		1,747,650		1,800,568
Securities not due on a single maturity date	22,354		23,037		1,374,769		1,394,936
Other securities (no maturity)	—		—		931		931
Total	$	335,946	$	341,925	$	3,397,043	$	3,473,598

The carrying value, which approximates the fair value, of securities pledged as collateral, to secure public deposits and for other purposes, amounted to $2.0 billion at December 31, 2020 and $1.7 billion at December 31, 2019.

There were approximately $54.8 million of gross realized gains and $15,000 of gross realized losses from the sale of securities during the year ended December 31, 2020. During 2020, the Company sold approximately $1.7 billion of investment securities to create additional liquidity. There were approximately $13.3 million of gross realized gains and $4,000 of gross realized losses from the sale of securities during the year ended December 31, 2019. There were approximately $65,000 of gross realized gains and $4,000 of gross realized losses from the sale of securities during the year ended December 31, 2018. The income tax expense/benefit related to security gains/losses was 26.135% of the gross amounts in 2020, 2019 and 2018.