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Investment Securities
 3 Months Ended
Mar. 31, 2022
Investments, Debt and Equity Securities [Abstract]  
Investment Securities Investment Securities
The following table presents the amortized cost and estimated fair values of investment securities for the periods presented:
March 31, 2023
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair
Value
Available for Sale(dollars in thousands)
U.S. Government securities$225,844 $— $(5,791)$220,053 
U.S. Government-sponsored agency securities1,047 — (30)1,017 
State and municipal securities1,211,841 967 (145,210)1,067,598 
Corporate debt securities475,052 — (36,018)439,034 
Collateralized mortgage obligations141,587 — (11,137)130,450 
Residential mortgage-backed securities238,775 33 (26,234)212,574 
Commercial mortgage-backed securities651,767 — (80,104)571,663 
   Total $2,945,913 $1,000 $(304,524)$2,642,389 
Held to Maturity
Residential mortgage-backed securities$444,431 $— $(52,078)$392,353 
Commercial mortgage-backed securities863,281 — (130,607)732,674 
Total $1,307,712 $— $(182,685)$1,125,027 

December 31, 2022
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair
Value
Available for Sale(dollars in thousands)
U.S. Government securities$226,140 $— $(7,655)$218,485 
U.S. Government sponsored agency securities1,050 — (42)1,008 
State and municipal securities1,284,245 283 (178,816)1,105,712 
Corporate debt securities459,792 — (37,483)422,309 
Collateralized mortgage obligations147,155 — (13,122)134,033 
Residential mortgage-backed securities242,527 18 (29,847)212,698 
Commercial mortgage-backed securities631,604 — (79,082)552,522 
   Total $2,992,513 $301 $(346,047)$2,646,767 
Held to Maturity
Residential mortgage-backed securities$457,325 $— $(57,480)$399,845 
Commercial mortgage-backed securities863,931 — (138,727)725,204 
Total $1,321,256 $— $(196,207)$1,125,049 

On May 1, 2022, the Corporation transferred certain residential mortgage-backed securities and commercial mortgage-backed securities from AFS to HTM classification as permitted by ASU 2019-04 Codification Improvements to Topic 326, Financial Instruments—Credit Losses, Topic 815, Derivatives and Hedging, and Topic 825, Financial Instruments. The estimated fair value of the securities transferred was $415.2 million, and the amortized cost of the securities was $479.0 million.

Securities carried at $1.1 billion at March 31, 2023 and December 31, 2022 were pledged as collateral to secure public and trust deposits.
The amortized cost and estimated fair values of debt securities as of March 31, 2023, by contractual maturity, are shown in the following table. Actual maturities may differ from contractual maturities because borrowers may have the right to call or prepay with or without call or prepayment penalties.
March 31, 2023
Available for SaleHeld to Maturity
 Amortized
Cost		Estimated
Fair Value		Amortized
Cost		Estimated
Fair Value
 (dollars in thousands)
Due in one year or less$237,505 $231,571 $— $— 
Due from one year to five years75,121 72,835 — — 
Due from five years to ten years544,101 509,791 — — 
Due after ten years1,057,057 913,505 — — 
1,913,784 1,727,702 — — 
Residential mortgage-backed securities(1)
238,775 212,574 444,431 392,353 
Commercial mortgage-backed securities(1)
651,767 571,663 863,281 732,674 
Collateralized mortgage obligations(1)
141,587 130,450 — — 
  Total$2,945,913 $2,642,389 $1,307,712 $1,125,027 
(1) Maturities for mortgage-backed securities and collateralized mortgage obligations are dependent upon the interest rate environment and prepayments on the underlying loans.

The following table presents information related to gross realized gains and losses on the sales of securities for the periods presented:
Gross Realized GainsGross Realized LossesNet  Gains (Losses)
Three months ended(dollars in thousands)
March 31, 2023$283 $(260)$23 
March 31, 20221,546 (1,527)19 

The following tables present the gross unrealized losses and estimated fair values of investment securities, aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position for the periods presented:
March 31, 2023
Less than 12 months12 months or longerTotal
Number of SecuritiesEstimated
Fair Value		Unrealized
Losses		Number of SecuritiesEstimated
Fair Value		Unrealized
Losses		Estimated
Fair Value		Unrealized
Losses
Available for Sale(dollars in thousands)
U.S. Government securities1$97,566 $(2,210)2 $122,487 $(3,581)$220,053 $(5,791)
U.S. Government-sponsored agency securities11,017 (30)— — 1,017 (30)
State and municipal securities86 238,922 (6,326)258 735,187 (138,884)974,109 (145,210)
Corporate debt securities45 239,873 (13,138)27 196,406 (22,880)436,279 (36,018)
Collateralized mortgage obligations37 29,944 (1,499)59 100,506 (9,638)130,450 (11,137)
Residential mortgage-backed securities50 80,524 (4,743)25 129,035 (21,491)209,559 (26,234)
Commercial mortgage-backed securities75 138,365 (4,501)61 433,298 (75,603)571,663 (80,104)
Total available for sale295 $826,211 $(32,447)432 $1,716,919 $(272,077)$2,543,130 $(304,524)
Held to Maturity
Residential mortgage-backed securities106 $238,862 $(11,981)14 $153,491 $(40,097)$392,353 $(52,078)
Commercial mortgage-backed securities20 257,006 (17,272)40 475,668 (113,335)732,674 (130,607)
Total held to maturity126 $495,868 $(29,253)54 $629,159 $(153,432)$1,125,027 $(182,685)
December 31, 2022
Less than 12 months12 months or longerTotal
Number of SecuritiesEstimated
Fair Value		Unrealized
Losses		Number of SecuritiesEstimated
Fair Value		Unrealized
Losses		Estimated
Fair Value		Unrealized
Losses
Available for Sale(dollars in thousands)
U.S Government Securities1 $96,906 $(2,814)2 $121,579 $(4,841)$218,485 $(7,655)
U.S. Government sponsored agency securities1 1,008 (42)— — — 1,008 (42)
State and municipal securities360 995,122 (157,397)29 61,089 (21,419)1,056,211 (178,816)
Corporate debt securities66 376,398 (31,333)6 37,157 (6,150)413,555 (37,483)
Collateralized mortgage obligations96 113,191 (7,650)1 20,842 (5,472)134,033 (13,122)
Residential mortgage-backed securities81 154,861 (18,301)5 55,293 (11,546)210,154 (29,847)
Commercial mortgage-backed securities114 371,109 (38,845)20 181,413 (40,237)552,522 (79,082)
Total available for sale719 $2,108,595 $(256,382)63 $477,373 $(89,665)$2,585,968 $(346,047)
Held to Maturity
Residential mortgage-backed securities106 $246,667 $(14,275)14 $153,178 $(43,205)$399,845 $(57,480)
Commercial mortgage-backed securities21 258,255 (24,029)39 466,949 (114,698)725,204 (138,727)
    Total held to maturity127 $504,922 $(38,304)53 $620,127 $(157,903)$1,125,049 $(196,207)

The Corporation's collateralized mortgage obligations and mortgage-backed securities have contractual terms that generally do not permit the issuer to settle the securities at a price less than the amortized cost of the investment. The change in fair value of these securities is attributable to changes in interest rates and not credit quality. The Corporation does not have the intent to sell and does not believe it will more likely than not be required to sell any of these securities prior to a recovery of their fair value to amortized cost. Therefore, the Corporation does not have an ACL for these investments as of March 31, 2023 and December 31, 2022.

As of March 31, 2023 and December 31, 2022, no ACL was required for the Corporation's state and municipal securities. The Corporation does not have the intent to sell and does not believe it will more likely than not be required to sell any of these securities prior to a recovery of their fair value to amortized cost, which may be at maturity.
As of March 31, 2023 and December 31, 2022, all of the Corporation's corporate debt securities were rated above investment grade. Based on the payment status, rating and management's evaluation of these securities, no ACL was required for corporate debt securities as of March 31, 2023 or December 31, 2022.