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Investment Securities (Tables)
 12 Months Ended
Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]  
Schedule of Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Approximate Market Value of Securities
The amortized cost, gross unrealized gains, gross unrealized losses and approximate fair value of the Corporation's investment securities at the dates indicated were:
 Amortized CostGross Unrealized GainsGross Unrealized LossesFair Value
Available for sale at December 31, 2020 
U.S. Government-sponsored agency securities$2,380 $50 $— $2,430 
State and municipal1,168,711 89,420 246 1,257,885 
U.S. Government-sponsored mortgage-backed securities632,267 22,505 103 654,669 
Corporate obligations4,031 104 — 4,135 
Total available for sale1,807,389 112,079 349 1,919,119 
Held to maturity at December 31, 2020 
U.S. Government-sponsored agency securities31,087 10 113 30,984 
State and municipal619,927 34,978 32 654,873 
U.S. Government-sponsored mortgage-backed securities575,154 17,889 107 592,936 
Foreign investment1,500 — — 1,500 
Total held to maturity1,227,668 52,877 252 1,280,293 
Total Investment Securities$3,035,057 $164,956 $601 $3,199,412 


Amortized CostGross Unrealized GainsGross Unrealized LossesFair Value
Available for sale at December 31, 2019 
U.S. Government-sponsored agency securities$38,529 $346 $— $38,875 
State and municipal859,511 41,092 807 899,796 
U.S. Government-sponsored mortgage-backed securities842,349 10,378 1,404 851,323 
Corporate obligations31 — — 31 
Total available for sale1,740,420 51,816 2,211 1,790,025 
Held to maturity at December 31, 2019 
U.S. Government-sponsored agency securities15,619 1 37 15,583 
State and municipal354,115 15,151 107 369,159 
U.S. Government-sponsored mortgage-backed securities434,804 6,921 401 441,324 
Foreign investment1,500 — — 1,500 
Total held to maturity806,038 22,073 545 827,566 
Total Investment Securities2,546,458 73,889 2,756 2,617,591 
Investments' Gross Unrealized Losses and Fair Value Aggregated by Investment Category and Length of Time in Continuous Unrealized Loss Position
The following table shows the Corporation’s gross unrealized losses and fair value, aggregated by investment category and length of time the individual securities have been in a continuous unrealized loss position at December 31, 2020 and December 31, 2019

 Less than 12 Months12 Months or LongerTotal
 Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses
Temporarily Impaired Available for Sale Securities at December 31, 2020      
State and municipal$5,368 $246 $— $— $5,368 $246 
U.S. Government-sponsored mortgage-backed securities9,651 103 — — 9,651 103 
Total Temporarily Impaired Available for Sale Securities15,019 349 — — 15,019 349 
Temporarily Impaired Held to Maturity Securities at December 31, 2020      
U.S. Government-sponsored agency securities20,881 113 — — 20,881 113 
State and municipal3,477 32 — — 3,477 32 
U.S. Government-sponsored mortgage-backed securities41,504 107 — — 41,504 107 
Total Temporarily Impaired Held to Maturity Securities65,862 252 — — $65,862 252 
Total Temporarily Impaired Investment Securities$80,881 $601 $— $— $80,881 $601 
 Less than 12 Months12 Months or LongerTotal
 Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses
Temporarily Impaired Available for Sale Securities at December 31, 2019      
State and municipal$76,273 $807 $— $— $76,273 $807 
U.S. Government-sponsored mortgage-backed securities127,673 1,326 20,796 78 148,469 1,404 
Total Temporarily Impaired Available for Sale Securities203,946 2,133 20,796 78 224,742 2,211 
Temporarily Impaired Held to Maturity Securities at December 31, 2019      
U.S. Government-sponsored agency securities3,016 4 12,467 33 15,483 37 
State and municipal22,947 107 — — 22,947 107 
U.S. Government-sponsored mortgage-backed securities124,253 364 7,991 37 132,244 401 
Total Temporarily Impaired Held to Maturity Securities150,216 475 20,458 70 170,674 545 
Total Temporarily Impaired Investment Securities$354,162 $2,608 $41,254 $148 $395,416 $2,756 
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 securities available for sale and held to maturity 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.

 Available for SaleHeld to Maturity
 Amortized CostFair ValueAmortized CostFair Value
Maturity Distribution at December 31, 2020    
Due in one year or less$1,349 $1,353 $9,712 $9,755 
Due after one through five years5,545 5,764 22,241 23,190 
Due after five through ten years70,777 75,223 115,408 121,333 
Due after ten years1,097,451 1,182,110 505,153 533,079 
 1,175,122 1,264,450 652,514 687,357 
U.S. Government-sponsored mortgage-backed securities632,267 654,669 575,154 592,936 
Total Investment Securities$1,807,389 $1,919,119 $1,227,668 $1,280,293 

Available for SaleHeld to Maturity
Amortized CostFair ValueAmortized CostFair Value
Maturity Distribution at December 31, 2019
Due in one year or less$1,134 $1,136 $9,920 $10,105 
Due after one through five years5,031 5,141 45,197 45,654 
Due after five through ten years74,745 76,920 84,153 88,844 
Due after ten years817,161 855,505 231,964 241,639 
898,071 938,702 371,234 386,242 
U.S. Government-sponsored mortgage-backed securities842,349 851,323 434,804 441,324 
Total Investment Securities$1,740,420 $1,790,025 $806,038 $827,566 
Schedule of Gross Gains on Sales and Redemptions of Available for Sale Securities
Gross gains and losses on the sales and redemptions of available for sale securities for the years indicated are shown below.
202020192018
Sales and Redemptions of Available for Sale Securities:  
Gross gains$11,895 $4,415 $4,269 
Gross losses— — —