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Investment Securities
 9 Months Ended
Sep. 30, 2023
Investments, Debt and Equity Securities [Abstract]  
Investment Securities Investment Securities 
 
The following tables present the amortized cost, unrealized gains, unrealized losses, and approximate fair values of debt securities as of the dates presented:
September 30, 2023
 (in thousands) Amortized CostUnrealized GainsUnrealized LossesFair Value
Available for sale:    
U.S. Treasury and agencies$1,557,088 $2 $(120,538)$1,436,552 
Obligations of states and political subdivisions1,074,949 29 (76,180)998,798 
Mortgage-backed securities and collateralized mortgage obligations
6,748,366 69 (679,799)6,068,636 
Total available for sale securities$9,380,403 $100 $(876,517)$8,503,986 
Held to maturity:    
Mortgage-backed securities and collateralized mortgage obligations
$2,344 $714 $— $3,058 
Total held to maturity securities$2,344 $714 $— $3,058 


December 31, 2022
 (in thousands) 
Amortized CostUnrealized GainsUnrealized LossesFair Value
Available for sale:    
U.S. Treasury and agencies$1,035,532 $— $(99,358)$936,174 
Obligations of states and political subdivisions297,610 231 (28,041)269,800 
Mortgage-backed securities and collateralized mortgage obligations
2,405,139 3 (414,950)1,990,192 
Total available for sale securities$3,738,281 $234 $(542,349)$3,196,166 
Held to maturity:    
Mortgage-backed securities and collateralized mortgage obligations
$2,476 $721 $— $3,197 
Total held to maturity securities$2,476 $721 $— $3,197 

The Company elected to exclude accrued interest receivable from the amortized cost basis of debt securities disclosed throughout this note. Interest accrued on investment securities totaled $33.6 million and $10.6 million as of September 30, 2023 and December 31, 2022, respectively, and is included in Other Assets.
The following tables present debt securities that were in an unrealized loss position as of the dates presented, based on the length of time individual securities have been in an unrealized loss position.

September 30, 2023
Less than 12 Months12 Months or LongerTotal
  (in thousands) 
Fair ValueUnrealized LossesFair ValueUnrealized LossesFair ValueUnrealized Losses
Available for sale:      
U.S. Treasury and agencies$643,453 $11,464 $792,079 $109,074 $1,435,532 $120,538 
Obligations of states and political subdivisions
819,008 43,750 157,996 32,430 977,004 76,180 
Mortgage-backed securities and collateralized mortgage obligations
4,309,987 190,213 1,754,407 489,586 6,064,394 679,799 
Total temporarily impaired securities$5,772,448 $245,427 $2,704,482 $631,090 $8,476,930 $876,517 


December 31, 2022
Less than 12 Months12 Months or LongerTotal
  (in thousands)
Fair ValueUnrealized LossesFair ValueUnrealized LossesFair ValueUnrealized Losses
Available for sale:      
U.S. Treasury and agencies$734,473 $71,967 $201,701 $27,391 $936,174 $99,358 
Obligations of states and political subdivisions
160,078 10,037 60,381 18,004 220,459 28,041 
Mortgage-backed securities and collateralized mortgage obligations
592,032 61,813 1,398,061 353,137 1,990,093 414,950 
Total temporarily impaired securities$1,486,583 $143,817 $1,660,143 $398,532 $3,146,726 $542,349 

The number of individual debt securities in an unrealized loss position in the tables above increased to 1,490 at September 30, 2023, as compared to 473 at December 31, 2022. These unrealized losses on the debt securities held by the Company were caused by changes in market interest rates or the widening of market spreads subsequent to the initial purchase of these securities and are not due to the underlying credit of the issuers. Management monitors the published credit ratings of the issuers of the debt securities for material rating or outlook changes. Substantially all of the Company's obligations of states and political subdivisions are general obligation issuances. Substantially all of the available for sale mortgage-backed securities and collateralized mortgage obligations portfolio in an unrealized loss position at September 30, 2023 are issued or guaranteed by government sponsored enterprises. Because the decline in fair value of the debt securities is attributable to changes in interest rates or widening market spreads and not credit quality, these investments do not have an allowance for credit losses at September 30, 2023.

The following table presents the contractual maturities of debt securities at September 30, 2023:  

Available For SaleHeld To Maturity
  (in thousands) 
Amortized CostFair ValueAmortized CostFair Value
Due within one year$26,219 $26,100 $— $— 
Due after one year through five years1,974,579 1,909,880 3 3 
Due after five years through ten years2,591,611 2,406,645 3 13 
Due after ten years4,787,994 4,161,361 2,338 3,042 
Total securities$9,380,403 $8,503,986 $2,344 $3,058 
The following table presents, as of September 30, 2023, investment securities which were pledged to secure borrowings, public deposits, and repurchase agreements as permitted or required by law: 
 (in thousands)Amortized CostFair Value
To state and local governments to secure public deposits$1,505,158 $1,305,616 
To secure repurchase agreements392,939 348,187 
Other securities pledged 2,321,670 2,081,880 
Total pledged securities$4,219,767 $3,735,683