The following tables present the amortized cost, the gross unrealized gains and losses and the fair value of the Company’s debt securities:


	September 30, 2021
(Dollars in thousands)	Amortized Cost		Gross Unrealized Gains	Gross Unrealized Losses	Fair Value
Available-for-sale
U.S. government and federal agency	$	31,449	207	(283)	31,373
U.S. government sponsored enterprises	47,324		6	(279)	47,051
State and local governments	479,116		29,666	(91)	508,691
Corporate bonds	191,164		6,958	(3)	198,119
Residential mortgage-backed securities	5,498,676		13,982	(21,313)	5,491,345
Commercial mortgage-backed securities	1,081,008		36,640	(3,647)	1,114,001
Total available-for-sale	$	7,328,737	87,459	(25,616)	7,390,580
Held-to-maturity
State and local governments	$	1,128,299	20,131	(1,977)	1,146,453
Total held-to-maturity	$	1,128,299	20,131	(1,977)	1,146,453


	December 31, 2020
(Dollars in thousands)	Amortized Cost		Gross Unrealized Gains	Gross Unrealized Losses	Fair Value
Available-for-sale
U.S. government and federal agency	$	38,568	287	(267)	38,588
U.S. government sponsored enterprises	9,747		34	—	9,781
State and local governments	1,321,763		94,974	(54)	1,416,683
Corporate bonds	336,867		12,239	(8)	349,098
Residential mortgage-backed securities	2,261,463		27,631	(4)	2,289,090
Commercial mortgage-backed securities	1,177,458		57,575	(459)	1,234,574
Total available-for-sale	$	5,145,866	192,740	(792)	5,337,814
Held-to-maturity
State and local governments	$	189,836	13,380	—	203,216
Total held-to-maturity	$	189,836	13,380	—	203,216

The following table presents the amortized cost and fair value of available-for-sale and held-to-maturity debt securities by contractual maturity at September 30, 2021. Actual maturities may differ from expected or contractual maturities since some issuers have the right to prepay obligations with or without prepayment penalties.


	September 30, 2021
	Available-for-Sale			Held-to-Maturity
(Dollars in thousands)	Amortized Cost		Fair Value	Amortized Cost	Fair Value
Due within one year	$	68,088	68,877	1,529	1,544
Due after one year through five years	186,292		195,336	27,382	29,114
Due after five years through ten years	250,903		259,085	89,010	94,026
Due after ten years	243,770		261,936	1,010,378	1,021,769
	749,053		785,234	1,128,299	1,146,453
Mortgage-backed securities 1	6,579,684		6,605,346	—	—
Total	$	7,328,737	7,390,580	1,128,299	1,146,453

Proceeds from sales and calls of debt securities and the associated gains and losses that have been included in earnings are listed below:


	Three Months ended			Nine Months ended
(Dollars in thousands)	September 30, 2021		September 30, 2020	September 30, 2021	September 30, 2020
Available-for-sale
Proceeds from sales and calls of debt securities	$	74,328	69,304	151,025	184,088
Gross realized gains 1	507		102	945	1,206
Gross realized losses 1	(9)		(78)	(75)	(192)
Held-to-maturity
Proceeds from calls of debt securities	19,120		9,280	25,480	29,530
Gross realized gains 1	—		—	—	1
Gross realized losses 1	(666)		—	(815)	—

In assessing whether a credit loss existed on available-for-sale debt securities with unrealized losses, the Company compared the present value of cash flows expected to be collected from the debt securities with the amortized cost basis of the debt securities. In addition, the following factors were evaluated individually and collectively in determining the existence of expected credit losses:

•the overall deal and payment structure of the debt securities, including the investor entity’s position within the structure, underlying obligors, financial condition and near-term prospects of the issuer, including specific events which may affect the issuer’s operations or future earnings, and credit support or enhancements; and

The following table summarizes available-for-sale debt securities that were in an unrealized loss position for which an ACL has not been recorded, based on the length of time the individual securities have been in an unrealized loss position. The number of available-for-sale debt securities in an unrealized position is also disclosed.


	September 30, 2021
	Number of Securities	Less than 12 Months			12 Months or More		Total
(Dollars in thousands)		Fair Value		Unrealized Loss	Fair Value	Unrealized Loss	Fair Value	Unrealized Loss
Available-for-sale
U.S. government and federal agency	23	$	9,736	(78)	5,398	(205)	15,134	(283)
U.S. government sponsored enterprises	3	45,279		(279)	—	—	45,279	(279)
State and local governments	11	15,596		(30)	1,742	(61)	17,338	(91)
Corporate bonds	1	5,016		(3)	—	—	5,016	(3)
Residential mortgage-backed securities	115	3,730,195		(21,312)	26	(1)	3,730,221	(21,313)
Commercial mortgage-backed securities	21	79,332		(866)	114,338	(2,781)	193,670	(3,647)
Total available-for-sale	174	$	3,885,154	(22,568)	121,504	(3,048)	4,006,658	(25,616)
Held-to-maturity
State and local governments	126	$	317,479	(1,977)	—	—	317,479	(1,977)
Total held-to-maturity	126	$	317,479	(1,977)	—	—	317,479	(1,977)


	December 31, 2020
	Number of Securities	Less than 12 Months			12 Months or More		Total
(Dollars in thousands)		Fair Value		Unrealized Loss	Fair Value	Unrealized Loss	Fair Value	Unrealized Loss
Available-for-sale
U.S. government and federal agency	18	$	13,814	(258)	726	(9)	14,540	(267)

State and local governments	6	3,121		(54)	—	—	3,121	(54)
Corporate bonds	3	5,500		(8)	—	—	5,500	(8)
Residential mortgage-backed securities	14	2,354		(4)	27	—	2,381	(4)
Commercial mortgage-backed securities	5	120,741		(459)	—	—	120,741	(459)
Total available-for-sale	46	$	145,530	(783)	753	(9)	146,283	(792)

With respect to severity, the majority of available-for-sale debt securities with unrealized loss positions at September 30, 2021 have unrealized losses as a percentage of book value of less than five percent. A substantial portion of such securities were issued by Federal National Mortgage Association (“Fannie Mae”), Federal Home Loan Mortgage Corporation (“Freddie Mac”), Government National Mortgage Association (“Ginnie Mae”) and other agencies of the U.S. government or have credit ratings issued by one or more of the NRSRO entities in the four highest credit rating categories. All of the Company’s available-for-sale debt securities with unrealized loss positions at September 30, 2021 have been determined to be investment grade.

The Company did not have any past due available-for-sale debt securities as of September 30, 2021 and December 31, 2020, respectively. Accrued interest receivable on available-for-sale debt securities totaled $17,791,000 and $20,215,000 at September 30, 2021, and December 31, 2020, respectively, and was excluded from the estimate of credit losses.

Based on an analysis of its available-for-sale debt securities with unrealized losses as of September 30, 2021, the Company determined the decline in value was unrelated to credit losses and was primarily the result of changes in interest rates and market spreads subsequent to acquisition. The fair value of the debt securities is expected to recover as payments are received and the debt securities approach maturity. In addition, as of September 30, 2021, management determined it did not intend to sell available-for-sale debt securities with unrealized losses, and there was no expected requirement to sell such securities before recovery of their amortized cost. As a result, no ACL was recorded on available-for-sale debt securities at September 30, 2021. As part of this determination, the Company considered contractual obligations, regulatory constraints, liquidity, capital, asset/liability management and securities portfolio objectives and whether or not any of the Company’s investment securities were managed by third-party investment funds.

The Company measured expected credit losses on held-to-maturity debt securities on a collective basis by major security type and NRSRO credit ratings, which is the Company’s primary credit quality indicator for state and local government securities. The estimate of expected credit losses considered historical credit loss information that was adjusted for current conditions as well as reasonable and supportable forecasts. The following table summarizes the amortized cost of held-to-maturity debt securities aggregated by NRSRO credit rating:


(Dollars in thousands)	September 30, 2021		December 31, 2020
Held-to-maturity
S&P: AAA / Moody’s: Aaa	$	274,791	39,022
S&P: AA+, AA, AA- / Moody’s: Aa1, Aa2, Aa3	812,759		123,664
S&P: A+, A, A- / Moody’s: A1, A2, A3	38,739		27,150
S&P: BBB+, BBB, BBB- / Moody’s: Baa1, Baa2, Baa3	—		—
Not rated by either entity	2,010		—
Below investment grade	—		—
Total held-to-maturity	$	1,128,299	189,836

The Company’s held-to-maturity debt securities portfolio is primarily comprised of general obligation and revenue bonds with NRSRO ratings in the four highest credit rating categories. All of the Company’s held-to-maturity debt securities at September 30, 2021 have been determined to be investment grade.

As of September 30, 2021 and December 31, 2020, the Company did not have any held-to-maturity debt securities past due. Accrued interest receivable on held-to-maturity debt securities totaled $11,664,000 and $1,728,000 at September 30, 2021 and December 31, 2020, respectively, and were excluded from the estimate of credit losses.

Based on the Company’s evaluation, an insignificant amount of credit losses is expected on the held-to-maturity debt securities portfolio; therefore, no ACL was recorded at September 30, 2021 or December 31, 2020.