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Investment Securities
 9 Months Ended
Sep. 30, 2023
Investments, Debt and Equity Securities [Abstract]  
Investment Securities Investment Securities
The following table presents the amortized cost and estimated fair values of investment securities:
September 30, 2024
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair
Value
Available for Sale(dollars in thousands)
State and municipal securities$963,138 $814 $(112,619)$851,333 
Corporate debt securities318,631 473 (22,524)296,580 
Collateralized mortgage obligations746,328 11,858 (8,718)749,468 
Residential mortgage-backed securities911,713 6,349 (22,427)895,635 
Commercial mortgage-backed securities604,376 — (84,096)520,280 
   Total $3,544,186 $19,494 $(250,384)$3,313,296 
Held to Maturity
Residential mortgage-backed securities$373,313 $— $(44,387)$328,926 
Commercial mortgage-backed securities858,669 — (123,291)735,378 
Total $1,231,982 $— $(167,678)$1,064,304 

December 31, 2023
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair
Value
Available for Sale(dollars in thousands)
U.S. Government securities$42,475 $— $(314)$42,161 
U.S. Government-sponsored agency securities1,038 — (28)1,010 
State and municipal securities1,200,571 1,089 (129,647)1,072,013 
Corporate debt securities480,714 473 (40,636)440,551 
Collateralized mortgage obligations122,824 — (11,390)111,434 
Residential mortgage-backed securities223,273 7 (26,485)196,795 
Commercial mortgage-backed securities627,364 — (92,976)534,388 
   Total $2,698,259 $1,569 $(301,476)$2,398,352 
Held to Maturity
Residential mortgage-backed securities$407,075 $— $(51,805)$355,270 
Commercial mortgage-backed securities860,847 — (143,910)716,937 
Total $1,267,922 $— $(195,715)$1,072,207 

In May 2024, the Corporation sold $345.7 million AFS securities and recorded a pre-tax loss of $20.3 million. The proceeds from the sale were reinvested into higher-yielding securities of a similar type and similar duration.

Securities carried at $0.6 billion and $0.4 billion at September 30, 2024 and December 31, 2023, 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, 2024, by contractual maturity, are shown in the following table. Actual maturities may differ from contractual maturities because issuers may have the right to call or borrowers may have the right to prepay with or without call or prepayment penalties.
September 30, 2024
Available for SaleHeld to Maturity
 Amortized
Cost		Estimated
Fair Value		Amortized
Cost		Estimated
Fair Value
 (dollars in thousands)
Due in one year or less$9,727 $9,548 $— $— 
Due from one year to five years87,566 84,556 — — 
Due from five years to ten years340,494 319,163 — — 
Due after ten years843,982 734,646 — — 
1,281,769 1,147,913 — — 
Residential mortgage-backed securities(1)
911,713 895,635 373,313 328,926 
Commercial mortgage-backed securities(1)
604,376 520,280 858,669 735,378 
Collateralized mortgage obligations(1)
746,328 749,468 — — 
  Total$3,544,186 $3,313,296 $1,231,982 $1,064,304 
(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:
Gross Realized GainsGross Realized LossesNet Gains (Losses)
Three months ended(dollars in thousands)
September 30, 2024$88 $(89)$(1)
September 30, 2023— — — 
Nine months ended
September 30, 2024$179 $(20,462)$(20,283)
September 30, 2023283 (264)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:
September 30, 2024
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)
State and municipal securities7 $19,919 $(138)273 $790,484 $(112,481)$810,403 $(112,619)
Corporate debt securities— — — 49 271,021 (22,524)271,021 (22,524)
Collateralized mortgage obligations2 39,917 (181)78 91,327 (8,537)131,244 (8,718)
Residential mortgage-backed securities8 165,584 (486)69 187,709 (21,941)353,293 (22,427)
Commercial mortgage-backed securities— — — 135 520,280 (84,096)520,280 (84,096)
Total available for sale17 $225,420 $(805)604 $1,860,821 $(249,579)$2,086,241 $(250,384)
Held to Maturity
Residential mortgage-backed securities— $— $— 120 $328,926 $(44,387)$328,926 $(44,387)
Commercial mortgage-backed securities— — — 60 735,378 (123,291)735,378 (123,291)
Total held to maturity— $— $— 180 $1,064,304 $(167,678)$1,064,304 $(167,678)

December 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 Securities— $— $— 1 $42,161 $(314)$42,161 $(314)
U.S. Government-sponsored agency securities— — — 1 1,010 (28)1,010 (28)
State and municipal securities40 76,155 (858)314 917,274 (128,789)993,429 (129,647)
Corporate debt securities8 42,945 (1,326)60 370,523 (39,310)413,468 (40,636)
Collateralized mortgage obligations— — — 93 111,434 (11,390)111,434 (11,390)
Residential mortgage-backed securities6 409 (3)69 195,453 (26,482)195,862 (26,485)
Commercial mortgage-backed securities2 26,907 (1,053)133 507,481 (91,923)534,388 (92,976)
Total available for sale56 $146,416 $(3,240)671 $2,145,336 $(298,236)$2,291,752 $(301,476)
Held to Maturity
Residential mortgage-backed securities— $— $— 60 $355,270 $(51,805)$355,270 $(51,805)
Commercial mortgage-backed securities— — — 120 716,937 (143,910)716,937 (143,910)
    Total held to maturity— $— $— 180 $1,072,207 $(195,715)$1,072,207 $(195,715)

The Corporation's collateralized mortgage obligations, residential mortgage-backed securities and commercial 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 did not record a loss on these investments as of September 30, 2024 and December 31, 2023.

As of September 30, 2024 and December 31, 2023, 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. Therefore, the Corporation did not record a loss on these investments as of September 30, 2024 and December 31, 2023.

The majority of the corporate debt securities were rated at or above investment grade as of September 30, 2024 and December 31, 2023. Based on the payment status, rating and management's evaluation of these securities, no ACL was required for corporate debt securities as of September 30, 2024 and December 31, 2023. The Corporation does not have the intent to sell and
does not believe it will more likely than not to be required to sell any of these securities prior to a recovery of their fair value to amortized cost, which may be at maturity. Therefore, the Corporation did not record a loss on these investments as of September 30, 2024 and December 31, 2023.