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INVESTMENT SECURITIES (Tables)
 3 Months Ended
Mar. 31, 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 table summarizes the amortized cost, gross unrealized gains and losses and approximate fair value of investment securities available for sale as of March 31, 2024 and December 31, 2023.

 Amortized
Cost		Gross Unrealized
Gains		Gross Unrealized
Losses		Fair
Value
Available for sale at March 31, 2024    
U.S. Government-sponsored agency securities$110,457 $— $17,325 $93,132 
State and municipal1,179,288 27 136,252 1,043,063 
U.S. Government-sponsored mortgage-backed securities565,177 336 93,469 472,044 
Corporate obligations12,950 — 976 11,974 
Total available for sale$1,867,872 $363 $248,022 $1,620,213 

 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 table summarizes the amortized cost, gross unrealized gains and losses, approximate fair value and allowance for credit losses on investment securities held to maturity as of March 31, 2024 and December 31, 2023.

Amortized
Cost		Allowance for Credit LossesNet Carrying AmountGross Unrealized
Gains		Gross Unrealized
Losses		Fair
Value
Held to maturity at March 31, 2024   
U.S. Government-sponsored agency securities$370,310 $— $370,310 $— $66,154 $304,156 
State and municipal1,097,848 245 1,097,603 691 171,353 927,186 
U.S. Government-sponsored mortgage-backed securities693,948 — 693,948 — 106,317 587,631 
Foreign investment1,500 — 1,500 — 22 1,478 
Total held to maturity$2,163,606 $245 $2,163,361 $691 $343,846 $1,820,451 


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 March 31, 2024, aggregated by credit quality indicator.
Held to Maturity
State and municipalOtherTotal
Credit Rating:
Aaa$114,449 $70,587 $185,036 
Aa1159,923 — 159,923 
Aa2169,157 — 169,157 
Aa3133,379 — 133,379 
A1131,318 — 131,318 
A216,837 — 16,837 
A33,468 — 3,468 
Non-rated369,317 995,171 1,364,488 
Total$1,097,848 $1,065,758 $2,163,606 
Summary of Investment Securities in a Continuous Unrealized Loss Position
The following tables summarize, as of March 31, 2024 and December 31, 2023, investment securities available for sale in an unrealized loss position for which an allowance for credit losses has not been recorded, aggregated by security type and length of time in a continuous unrealized loss position.
Less than 12 Months12 Months or LongerTotal
Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses
Investment securities available for sale at March 31, 2024
U.S. Government-sponsored agency securities$— $— $93,132 $17,325 $93,132 $17,325 
State and municipal54,062 1,643 980,756 134,609 1,034,818 136,252 
U.S. Government-sponsored mortgage-backed securities43,251 499 408,518 92,970 451,769 93,469 
Corporate obligations— — 11,944 976 11,944 976 
Total investment securities available for sale$97,313 $2,142 $1,494,350 $245,880 $1,591,663 $248,022 

 
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 table summarizes 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 March 31, 2024
U.S. Government-sponsored agency securities$17,325 14
State and municipal136,252 721
U.S. Government-sponsored mortgage-backed securities93,469 156
Corporate obligations976 10
Total investment securities available for sale$248,022 901 
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.
March 31, 2024December 31, 2023
Investments available for sale reported at less than historical cost:  
Historical cost$1,839,685 $1,781,108 
Fair value1,591,663 1,560,554 
Gross unrealized losses$248,022 $220,554 
Percent of the Corporation's investments available for sale98.2 %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 March 31, 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 March 31, 2024    
Due in one year or less$1,865 $1,855 $12,971 $12,831 
Due after one through five years24,392 22,654 112,676 105,283 
Due after five through ten years141,962 131,832 137,043 125,783 
Due after ten years1,134,476 991,828 1,206,968 988,923 
 1,302,695 1,148,169 1,469,658 1,232,820 
U.S. Government-sponsored mortgage-backed securities565,177 472,044 693,948 587,631 
Total investment securities$1,867,872 $1,620,213 $2,163,606 $1,820,451 

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 $1,870,374 
Summary of Gross Gains and Losses on Sales and Redemptions of Available for Sale Securities
Gross gains and losses on the sales and redemptions of investment securities available for sale for the three months ended March 31, 2024 and 2023 are shown below.
Three Months Ended March 31,
20242023
Sales and redemptions of investment securities available for sale:  
Gross gains$— $12 
Gross losses(2)(1,583)
Net gains (losses) on sales and redemptions of investment securities available for sale$(2)$(1,571)