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Investment Securities
 9 Months Ended
Sep. 30, 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:
September 30, 2023
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair
Value
Available for Sale(dollars in thousands)
U.S. Government securities$225,244 $— $(2,798)$222,446 
U.S. Government-sponsored agency securities1,041 — (54)987 
State and municipal securities1,203,732 — (254,586)949,146 
Corporate debt securities479,941 320 (51,777)428,484 
Collateralized mortgage obligations127,845 — (15,628)112,217 
Residential mortgage-backed securities228,431 5 (39,161)189,275 
Commercial mortgage-backed securities632,334 — (116,021)516,313 
   Total $2,898,568 $325 $(480,025)$2,418,868 
Held to Maturity
Residential mortgage-backed securities$418,362 $— $(73,552)$344,810 
Commercial mortgage-backed securities861,371 — (186,337)675,034 
Total $1,279,733 $— $(259,889)$1,019,844 
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 $0.8 billion and $1.1 billion at September 30, 2023 and December 31, 2022, respectively, were pledged as collateral to secure public and trust deposits.

The amortized cost and estimated fair values of debt securities as of September 30, 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.
September 30, 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$231,703 $228,876 $— $— 
Due from one year to five years111,296 106,291 — — 
Due from five years to ten years550,997 487,118 — — 
Due after ten years1,015,962 778,778 — — 
1,909,958 1,601,063 — — 
Residential mortgage-backed securities(1)
228,431 189,275 418,362 344,810 
Commercial mortgage-backed securities(1)
632,334 516,313 861,371 675,034 
Collateralized mortgage obligations(1)
127,845 112,217 — — 
  Total$2,898,568 $2,418,868 $1,279,733 $1,019,844 
(1) Maturities for mortgage-backed securities and collateralized mortgage obligations are dependent upon the interest rate environment and prepayments on the
underlying loans.
The following table presents information related to gross realized gains and losses on the sales of securities for the periods presented:
Gross Realized GainsGross Realized LossesNet  Gains (Losses)
Three months ended(dollars in thousands)
September 30, 2023$— $— $— 
September 30, 202233 (86)(53)
Nine months ended
September 30, 2023$283 $(264)$19 
September 30, 20221,587 (1,613)(26)

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:
September 30, 2023
Less than 12 months12 months or longerTotal
Estimated
Fair Value		Unrealized
Losses		Estimated
Fair Value		Unrealized
Losses		Estimated
Fair Value		Unrealized
Losses
Available for Sale(dollars in thousands)
U.S. Government securities$— $— $222,446 $(2,798)$222,446 $(2,798)
U.S. Government-sponsored agency securities987 (54)— — 987 (54)
State and municipal securities138,638 (13,674)807,811 (240,912)946,449 (254,586)
Corporate debt securities118,791 (10,241)300,480 (41,536)419,271 (51,777)
Collateralized mortgage obligations10,292 (1,397)101,925 (14,231)112,217 (15,628)
Residential mortgage-backed securities32,935 (4,272)155,351 (34,889)188,286 (39,161)
Commercial mortgage-backed securities67,258 (6,622)449,055 (109,399)516,313 (116,021)
Total available for sale$368,901 $(36,260)$2,037,068 $(443,765)$2,405,969 $(480,025)
Held to Maturity
Residential mortgage-backed securities$— $— $344,810 $(73,552)$344,810 $(73,552)
Commercial mortgage-backed securities— — 675,034 (186,337)675,034 (186,337)
Total held to maturity$— $— $1,019,844 $(259,889)$1,019,844 $(259,889)
December 31, 2022
Less than 12 months12 months or longerTotal
Estimated
Fair Value		Unrealized
Losses		Estimated
Fair Value		Unrealized
Losses		Estimated
Fair Value		Unrealized
Losses
Available for Sale(dollars in thousands)
U.S. Government Securities$96,906 $(2,814)$121,579 $(4,841)$218,485 $(7,655)
U.S. Government sponsored agency securities1,008 (42)— — 1,008 (42)
State and municipal securities995,122 (157,397)61,089 (21,419)1,056,211 (178,816)
Corporate debt securities376,398 (31,333)37,157 (6,150)413,555 (37,483)
Collateralized mortgage obligations113,191 (7,650)20,842 (5,472)134,033 (13,122)
Residential mortgage-backed securities154,861 (18,301)55,293 (11,546)210,154 (29,847)
Commercial mortgage-backed securities371,109 (38,845)181,413 (40,237)552,522 (79,082)
Total available for sale$2,108,595 $(256,382)$477,373 $(89,665)$2,585,968 $(346,047)
Held to Maturity
Residential mortgage-backed securities$246,667 $(14,275)$153,178 $(43,205)$399,845 $(57,480)
Commercial mortgage-backed securities258,255 (24,029)466,949 (114,698)725,204 (138,727)
    Total held to maturity$504,922 $(38,304)$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 September 30, 2023 and December 31, 2022.

As of September 30, 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 September 30, 2023 and December 31, 2022, no ACL was required for the Corporation’s corporate debt 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.