
	December 31, 2020
	Amortized Cost		Gross Unrealized		Fair Value
(Dollars in thousands)			Gains	Losses
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
Total debt securities	$	5,335,702	206,120	(792)	5,541,030


	December 31, 2019
	Amortized Cost		Gross Unrealized		Fair Value
(Dollars in thousands)			Gains	Losses
Available-for-sale
U.S. government and federal agency	$	20,061	48	(65)	20,044
U.S. government sponsored enterprises	42,724		953	—	43,677
State and local governments	679,784		22,694	(80)	702,398
Corporate bonds	155,665		1,938	(1)	157,602
Residential mortgage-backed securities	731,766		7,507	(549)	738,724
Commercial mortgage-backed securities	891,374		22,825	(1,392)	912,807
Total available-for-sale	2,521,374		55,965	(2,087)	2,575,252
Held-to-maturity
State and local governments	224,611		9,785	—	234,396
Total held-to-maturity	224,611		9,785	—	234,396
Total debt securities	$	2,745,985	65,750	(2,087)	2,809,648

The following table presents the amortized cost and fair value of available-for-sale and held-to-maturity debt securities by contractual maturity at December 31, 2020. Actual maturities may differ from expected or contractual maturities since some issuers have the right to prepay obligations with or without prepayment penalties.


	December 31, 2020
	Available-for-Sale			Held-to-Maturity
(Dollars in thousands)	Amortized Cost		Fair Value	Amortized Cost	Fair Value
Due within one year	$	128,263	129,346	484	489
Due after one year through five years	246,473		258,040	22,032	23,625
Due after five years through ten years	266,248		280,021	67,678	73,367
Due after ten years	1,065,961		1,146,743	99,642	105,735
	1,706,945		1,814,150	189,836	203,216
Mortgage-backed securities 1	3,438,921		3,523,664	—	—
Total	$	5,145,866	5,337,814	189,836	203,216


	Years ended
(Dollars in thousands)	December 31, 2020		December 31, 2019	December 31, 2018
Available-for-sale
Proceeds from sales and calls of debt securities	$	240,521	928,710	265,587
Gross realized gains 1	1,400		18,936	443
Gross realized losses 1	(262)		(4,513)	(455)
Held-to-maturity
Proceeds from calls of debt securities	32,735		58,750	79,000
Gross realized gains 1	1		2	101
Gross realized losses 1	—		(10)	(1,202)

At December 31, 2020 and 2019, the Company had debt securities with carrying values of $2,169,967,000 and $1,475,752,000, respectively, pledged as collateral for FHLB advances, FRB discount window borrowings, securities sold under agreements to repurchase (“repurchase agreements”), and deposits of several state and local government units.

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 potential 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 loss position is also disclosed.


	December 31, 2020
	Number	Less than 12 Months			12 Months or More		Total
(Dollars in thousands)	of Securities	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)


	December 31, 2019
	Number	Less than 12 Months			12 Months or More		Total
(Dollars in thousands)	of Securities	Fair Value		Unrealized Loss	Fair Value	Unrealized Loss	Fair Value	Unrealized Loss
Available-for-sale
U.S. government and federal agency	20	$	464	—	9,902	(65)	10,366	(65)

State and local governments	12	19,044		(80)	—	—	19,044	(80)
Corporate bonds	2	7,378		(1)	—	—	7,378	(1)
Residential mortgage-backed securities	35	85,562		(234)	29,038	(315)	114,600	(549)
Commercial mortgage-backed securities	19	177,051		(1,293)	7,697	(99)	184,748	(1,392)
Total available-for-sale	88	$	289,499	(1,608)	46,637	(479)	336,136	(2,087)

With respect to severity, the majority of available-for-sale debt securities with unrealized loss positions at December 31, 2020 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 December 31, 2020 have been determined to be investment grade.

As of December 31, 2020, the Company did not have any past due available-for-sale debt securities. Accrued interest receivable on available-for-sale debt securities totaled $20,215,000 at December 31, 2020 and was excluded from the estimate of credit losses and there was no interest reversed during the year.

During the period ended December 31, 2020, the Company acquired available-for-sale debt securities from the secondary market and through the SBAZ acquisition. Such securities were evaluated and it was determined there were no PCD securities, so no allowance for credit losses was recorded.

Based on an analysis of its available-for-sale debt securities with unrealized losses as of December 31, 2020, 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 December 31, 2020, 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 December 31, 2020. 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)	December 31, 2020		December 31, 2019
Held-to-maturity
S&P: AAA / Moody’s: Aaa	$	39,022	65,217
S&P: AA+, AA, AA- / Moody’s: Aa1, Aa2, Aa3	123,664		130,316
S&P: A+, A, A- / Moody’s: A1, A2, A3	27,150		28,689

Not rated by either entity	—		389

Total held-to-maturity	$	189,836	224,611

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 December 31, 2020 have been determined to be investment grade.

As of 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 $1,728,000 at December 31, 2020 and was excluded from the estimate of credit losses.