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INVESTMENT SECURITIES (Tables)
 9 Months Ended
Sep. 30, 2023
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 September 30, 2023 and December 31, 2022.

 Amortized
Cost		Gross Unrealized
Gains		Gross Unrealized
Losses		Fair
Value
Available for sale at September 30, 2023    
U.S. Treasury$1,737 $— $24 $1,713 
U.S. Government-sponsored agency securities114,414 — 20,829 93,585 
State and municipal1,217,293 — 244,237 973,056 
U.S. Government-sponsored mortgage-backed securities536,530 — 116,567 419,963 
Corporate obligations12,960 — 1,141 11,819 
Total available for sale$1,882,934 $— $382,798 $1,500,136 

 Amortized
Cost		Gross Unrealized
Gains		Gross Unrealized
Losses		Fair
Value
Available for sale at December 31, 2022    
U.S. Treasury$2,501 $— $42 $2,459 
U.S. Government-sponsored agency securities119,154 — 17,192 101,962 
State and municipal1,530,048 438 178,726 1,351,760 
U.S. Government-sponsored mortgage-backed securities608,630 1 100,358 508,273 
Corporate obligations13,014 — 807 12,207 
Total available for sale$2,273,347 $439 $297,125 $1,976,661 
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 September 30, 2023 and December 31, 2022.

Amortized
Cost		Allowance for Credit LossesNet Carrying AmountGross Unrealized
Gains		Gross Unrealized
Losses		Fair
Value
Held to maturity at September 30, 2023   
U.S. Government-sponsored agency securities$378,706 $— $378,706 $— $78,882 $299,824 
State and municipal1,107,325 245 1,107,080 14 258,536 848,803 
U.S. Government-sponsored mortgage-backed securities726,302 — 726,302 — 135,163 591,139 
Foreign investment1,500 — 1,500 — 41 1,459 
Total held to maturity$2,213,833 $245 $2,213,588 $14 $472,622 $1,741,225 


Amortized
Cost		Allowance for Credit LossesNet Carrying AmountGross Unrealized
Gains		Gross Unrealized
Losses		Fair
Value
Held to maturity at December 31, 2022    
U.S. Government-sponsored agency securities$392,246 $— $392,246 $— $69,147 $323,099 
State and municipal1,117,552 245 1,117,307 647 197,064 921,135 
U.S. Government-sponsored mortgage-backed securities776,074 — 776,074 — 113,915 662,159 
Foreign investment1,500 — 1,500 — 28 1,472 
Total held to maturity$2,287,372 $245 $2,287,127 $647 $380,154 $1,907,865 
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, 2023, aggregated by credit quality indicator.
Held to Maturity
State and municipalOtherTotal
Credit Rating:
Aaa$114,709 $70,585 $185,294 
Aa1153,408 — 153,408 
Aa2181,369 — 181,369 
Aa3133,499 — 133,499 
A1131,310 — 131,310 
A210,172 — 10,172 
A310,125 — 10,125 
Non-rated372,733 1,035,923 1,408,656 
Total$1,107,325 $1,106,508 $2,213,833 
Summary of Investment Securities in a Continuous Unrealized Loss Position
The following tables summarize, as of September 30, 2023 and December 31, 2022, 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, 2023
U.S. Treasury$397 $1 $1,316 $23 $1,713 $24 
U.S. Government-sponsored agency securities796 3 92,789 20,826 93,585 20,829 
State and municipal86,964 10,369 886,092 233,868 973,056 244,237 
U.S. Government-sponsored mortgage-backed securities11,273 453 408,686 116,114 419,959 116,567 
Corporate obligations— — 11,788 1,141 11,788 1,141 
Total investment securities available for sale$99,430 $10,826 $1,400,671 $371,972 $1,500,101 $382,798 

 
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, 2022
U.S. Treasury$2,459 $42 $— $— $2,459 $42 
U.S. Government-sponsored agency securities48,940 4,973 53,022 12,219 101,962 17,192 
State and municipal1,177,104 150,096 108,652 28,630 1,285,756 178,726 
U.S. Government-sponsored mortgage-backed securities182,700 16,910 325,455 83,448 508,155 100,358 
Corporate obligations12,176 807 — — 12,176 807 
Total investment securities available for sale$1,423,379 $172,828 $487,129 $124,297 $1,910,508 $297,125 
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 September 30, 2023
U.S. Treasury$24 3
U.S. Government-sponsored agency securities20,829 16
State and municipal244,237 743
U.S. Government-sponsored mortgage-backed securities116,567 164
Corporate obligations1,141 10
Total investment securities available for sale$382,798 936 

Gross
Unrealized
Losses		Number of Securities
Investment securities available for sale at December 31, 2022
U.S. Treasury$42 5
U.S. Government-sponsored agency securities17,192 16
State and municipal178,726 946
U.S. Government-sponsored mortgage-backed securities100,358 177
Corporate obligations807 10
Total investment securities available for sale$297,125 1,154 
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, 2023December 31, 2022
Investments available for sale reported at less than historical cost:  
Historical cost$1,882,899 $2,207,633 
Fair value1,500,101 1,910,508 
Gross unrealized losses$382,798 $297,125 
Percent of the Corporation's investments available for sale99.99 %96.65 %
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, 2023 and December 31, 2022, 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, 2023    
Due in one year or less$3,403 $3,356 $9,422 $9,397 
Due after one through five years27,736 25,266 116,894 106,277 
Due after five through ten years127,442 110,668 134,603 117,134 
Due after ten years1,187,823 940,883 1,226,612 917,278 
 1,346,404 1,080,173 1,487,531 1,150,086 
U.S. Government-sponsored mortgage-backed securities536,530 419,963 726,302 591,139 
Total investment securities$1,882,934 $1,500,136 $2,213,833 $1,741,225 

Available for SaleHeld to Maturity
Amortized CostFair ValueAmortized CostFair Value
Maturity Distribution at December 31, 2022    
Due in one year or less$2,822 $2,809 $13,697 $13,749 
Due after one through five years11,694 11,265 80,697 76,453 
Due after five through ten years169,729 161,211 147,078 135,027 
Due after ten years1,480,472 1,293,103 1,269,826 1,020,477 
 1,664,717 1,468,388 1,511,298 1,245,706 
U.S. Government-sponsored mortgage-backed securities608,630 508,273 776,074 662,159 
Total investment securities$2,273,347 $1,976,661 $2,287,372 $1,907,865 
Summary of Gross Gains and Losses on Sales and Redemptions of Available for Sale Securities
Gross gains and losses on the sales and redemptions of investment securities available for sale for the three and nine months ended September 30, 2023 and 2022 are shown below.
Three Months Ended
September 30,		Nine Months Ended
September 30,
2023202220232022
Sales and redemptions of investment securities available for sale:    
Gross gains$— $495 $759 $1,176 
Gross losses(1,650)(14)(5,372)(39)
Net gains (losses) on sales and redemptions of investment securities available for sale$(1,650)$481 $(4,613)$1,137