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INVESTMENT SECURITIES (Tables)
 9 Months Ended
Sep. 30, 2024
Investments, Debt and Equity Securities [Abstract]  
Summary of Amortized Cost, Gross Unrealized Gains and Losses and Approximate Fair Value of Investment Securities
The following tables summarize the amortized cost, gross unrealized gains and losses and approximate fair value of investment securities available for sale as of September 30, 2024 and December 31, 2023.

 Amortized
Cost		Gross Unrealized
Gains		Gross Unrealized
Losses		Fair
Value
Available for sale at September 30, 2024    
U.S. Government-sponsored agency securities$97,670 $— $12,637 $85,033 
State and municipal1,090,953 95 106,937 984,111 
U.S. Government-sponsored mortgage-backed securities541,099 2,447 71,500 472,046 
Corporate obligations12,957 — 651 12,306 
Total available for sale$1,742,679 $2,542 $191,725 $1,553,496 
 Amortized
Cost		Gross Unrealized
Gains		Gross Unrealized
Losses		Fair
Value
Available for sale at December 31, 2023    
U.S. Government-sponsored agency securities$111,521 $— $16,214 $95,307 
State and municipal1,181,029 364 116,222 1,065,171 
U.S. Government-sponsored mortgage-backed securities541,343 462 86,990 454,815 
Corporate obligations12,947 — 1,128 11,819 
Total available for sale$1,846,840 $826 $220,554 $1,627,112 

The following tables summarize the amortized cost, gross unrealized gains and losses, approximate fair value and allowance for credit losses on investment securities held to maturity as of September 30, 2024 and December 31, 2023.

Amortized
Cost		Allowance for Credit LossesNet Carrying AmountGross Unrealized
Gains		Gross Unrealized
Losses		Fair
Value
Held to maturity at September 30, 2024   
U.S. Government-sponsored agency securities$361,335 $— $361,335 $— $53,432 $307,903 
State and municipal1,087,980 245 1,087,735 1,157 151,227 937,910 
U.S. Government-sponsored mortgage-backed securities658,079 — 658,079 — 82,382 575,697 
Foreign investment1,500 — 1,500 — 4 1,496 
Total held to maturity$2,108,894 $245 $2,108,649 $1,157 $287,045 $1,823,006 


Amortized
Cost		Allowance for Credit LossesNet Carrying AmountGross Unrealized
Gains		Gross Unrealized
Losses		Fair
Value
Held to maturity at December 31, 2023    
U.S. Government-sponsored agency securities$374,002 $— $374,002 $— $64,159 $309,843 
State and municipal1,099,201 245 1,098,956 1,625 152,113 948,713 
U.S. Government-sponsored mortgage-backed securities709,794 — 709,794 — 99,448 610,346 
Foreign investment1,500 — 1,500 — 28 1,472 
Total held to maturity$2,184,497 $245 $2,184,252 $1,625 $315,748 $1,870,374 
Summary of Amortized Cost of Investment Securities Held to Maturity Aggregated by Credit Quality Indicator
On a quarterly basis, the Corporation monitors the credit quality of investment securities held to maturity through the use of credit ratings. The following table summarizes the amortized cost of investment securities held to maturity at September 30, 2024, aggregated by credit quality indicator.
Held to Maturity
U.S. Government-sponsored agency securities (1)
State and municipal
U.S. Government-sponsored mortgage-backed securities (1)
Foreign investmentTotal
Credit Rating:
Aaa$361,335 $113,233 $658,079 $— $1,132,647 
Aa1— 160,281 — — 160,281 
Aa2— 177,858 — — 177,858 
Aa3— 112,393 — — 112,393 
A1— 141,608 — — 141,608 
A2— 20,313 — — 20,313 
Non-rated— 362,294 — 1,500 363,794 
Total$361,335 $1,087,980 $658,079 $1,500 $2,108,894 


(1) U.S. Government agency securities and U.S. Government mortgage-backed securities are included within the Aaa credit rating category due to their explicit or implicit government guarantees, which provide a high level of assurance regarding the timely collection of principal and interest payments.
Summary of Investment Securities in a Continuous Unrealized Loss Position
The following tables summarize, as of September 30, 2024 and December 31, 2023, investment securities available for sale in an unrealized loss position for which an allowance for credit losses has not been recorded, aggregated by security type and length of time in a continuous unrealized loss position.
Less than 12 Months12 Months or LongerTotal
Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses
Investment securities available for sale at September 30, 2024
U.S. Government-sponsored agency securities$— $— $85,033 $12,637 $85,033 $12,637 
State and municipal40,136 1,335 934,947 105,602 975,083 106,937 
U.S. Government-sponsored mortgage-backed securities18,916 77 346,666 71,423 365,582 71,500 
Corporate obligations— — 12,275 651 12,275 651 
Total investment securities available for sale$59,052 $1,412 $1,378,921 $190,313 $1,437,973 $191,725 
Less than 12 Months12 Months or LongerTotal
Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses
Investment securities available for sale at December 31, 2023
U.S. Government-sponsored agency securities$— $— $95,307 $16,214 $95,307 $16,214 
State and municipal55,514 1,076 963,584 115,146 1,019,098 116,222 
U.S. Government-sponsored mortgage-backed securities11,493 25 422,868 86,965 434,361 86,990 
Corporate obligations— — 11,788 1,128 11,788 1,128 
Total investment securities available for sale$67,007 $1,101 $1,493,547 $219,453 $1,560,554 $220,554 
The following tables summarize investment securities available for sale in an unrealized loss position for which an allowance for credit losses has not been recorded, aggregated by security type and the number of securities in the portfolio as of the dates indicated.

Gross
Unrealized
Losses		Number of Securities
Investment securities available for sale at September 30, 2024
U.S. Government-sponsored agency securities$12,637 13
State and municipal106,937 662
U.S. Government-sponsored mortgage-backed securities71,500 108
Corporate obligations651 10
Total investment securities available for sale$191,725 793 
Gross
Unrealized
Losses		Number of Securities
Investment securities available for sale at December 31, 2023
U.S. Government-sponsored agency securities$16,214 14
State and municipal116,222 691
U.S. Government-sponsored mortgage-backed securities86,990 150
Corporate obligations1,128 10
Total investment securities available for sale$220,554 865 
Summary of Investments in Debt and Equity Securities Reported in the Financial Statements at an Amount Less Than Their Historical Cost
Certain investment securities available for sale are reported in the financial statements at an amount less than their historical cost as indicated in the table below.
September 30, 2024December 31, 2023
  
Historical cost$1,629,698 $1,781,108 
Fair value1,437,973 1,560,554 
Gross unrealized losses$191,725 $220,554 
Percentage of the Corporation's investment securities available for sale in an unrealized loss position92.6 %95.9 %
Summary of Amortized Cost and Fair Value of Available for Sale Securities and Held to Maturity Securities by Contractual Maturity
The amortized cost and fair value of investment securities available for sale and held to maturity at September 30, 2024 and December 31, 2023, by contractual maturity are shown below. Expected maturities will differ from contractual maturities because issuers may have the right to call or prepay obligations with or without call or prepayment penalties. Securities not due at a single maturity are shown separately.

 Available for SaleHeld to Maturity
 Amortized CostFair ValueAmortized CostFair Value
Maturity Distribution at September 30, 2024    
Due in one year or less$1,809 $1,800 $15,139 $15,096 
Due after one through five years12,818 12,578 121,204 116,480 
Due after five through ten years162,996 151,369 165,418 151,803 
Due after ten years1,023,957 915,703 1,149,054 963,930 
 1,201,580 1,081,450 1,450,815 1,247,309 
U.S. Government-sponsored mortgage-backed securities541,099 472,046 658,079 575,697 
Total investment securities$1,742,679 $1,553,496 $2,108,894 $1,823,006 

Available for SaleHeld to Maturity
Amortized CostFair ValueAmortized CostFair Value
Maturity Distribution at December 31, 2023    
Due in one year or less$1,390 $1,382 $3,041 $3,043 
Due after one through five years24,899 23,372 118,592 111,723 
Due after five through ten years127,948 120,385 135,805 126,461 
Due after ten years1,151,260 1,027,158 1,217,265 1,018,801 
 1,305,497 1,172,297 1,474,703 1,260,028 
U.S. Government-sponsored mortgage-backed securities541,343 454,815 709,794 610,346 
Total investment securities$1,846,840 $1,627,112 $2,184,497 $1,870,374 
Summary of Gross Gains and Losses on Sales and Redemptions of Available for Sale Securities
Gross gains and losses on the sales of investment securities available for sale for the three and nine months ended September 30, 2024 and 2023 are shown below.
Three Months Ended September 30,Nine Months Ended September 30,
2024202320242023
Sales and redemptions of investment securities available for sale:    
Gross gains$7 $— $7 $759 
Gross losses(9,121)(1,650)(9,172)(5,372)
Net gains (losses) on sales and redemptions of investment securities available for sale$(9,114)$(1,650)$(9,165)$(4,613)