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Investment Securities
 3 Months Ended
Mar. 31, 2022
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 at March 31, 2022 and December 31, 2021: 
March 31, 2022
 (in thousands) Amortized CostUnrealized GainsUnrealized LossesFair Value
Available for sale:    
U.S. Treasury and agencies$893,811 $330 $(31,999)$862,142 
Obligations of states and political subdivisions317,743 2,463 (13,342)306,864 
Mortgage-backed securities and collateralized mortgage obligations
2,657,494 1,225 (189,645)2,469,074 
Total available for sale securities$3,869,048 $4,018 $(234,986)$3,638,080 
Held to maturity:    
Mortgage-backed securities and collateralized mortgage obligations
$2,700 $759 $— $3,459 
Total held to maturity securities$2,700 $759 $— $3,459 

December 31, 2021
 (in thousands) 
Amortized CostUnrealized GainsUnrealized LossesFair Value
Available for sale:    
U.S. Treasury and agencies$894,969 $27,279 $(4,195)$918,053 
Obligations of states and political subdivisions320,338 11,734 (1,288)330,784 
Mortgage-backed securities and collateralized mortgage obligations
2,649,048 19,093 (46,543)2,621,598 
Total available for sale securities$3,864,355 $58,106 $(52,026)$3,870,435 
Held to maturity:    
Mortgage-backed securities and collateralized mortgage obligations
$2,744 $770 $— $3,514 
Total held to maturity securities$2,744 $770 $— $3,514 

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 $12.6 million and $10.4 million as of March 31, 2022 and December 31, 2021, respectively, and is included in Other Assets.

Debt securities that were in an unrealized loss position as of March 31, 2022 and December 31, 2021 are presented in the following tables, based on the length of time individual securities have been in an unrealized loss position.

March 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$798,817 $28,447 $26,276 $3,552 $825,093 $31,999 
Obligations of states and political subdivisions
142,162 12,727 4,400 615 146,562 13,342 
Mortgage-backed securities and collateralized mortgage obligations
1,707,308 109,913 671,447 79,732 2,378,755 189,645 
Total temporarily impaired securities$2,648,287 $151,087 $702,123 $83,899 $3,350,410 $234,986 


December 31, 2021
Less than 12 Months12 Months or LongerTotal
  (in thousands)
Fair ValueUnrealized LossesFair ValueUnrealized LossesFair ValueUnrealized Losses
Available for sale:      
U.S. Treasury and agencies$197,529 $2,749 $28,378 $1,446 $225,907 $4,195 
Obligations of states and political subdivisions
75,200 1,153 2,162 135 77,362 1,288 
Mortgage-backed securities and collateralized mortgage obligations
1,777,288 40,579 129,943 5,964 1,907,231 46,543 
Total temporarily impaired securities$2,050,017 $44,481 $160,483 $7,545 $2,210,500 $52,026 

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. All of the available for sale mortgage-backed securities and collateralized mortgage obligations portfolio in an unrealized loss position at March 31, 2022 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 March 31, 2022.
The following table presents the contractual maturities of debt securities at March 31, 2022:  

Available For SaleHeld To Maturity
  (in thousands) 
Amortized CostFair ValueAmortized CostFair Value
Due within one year$11,321 $11,379 $— $— 
Due after one year through five years307,338 300,748 2 2 
Due after five years through ten years918,444 883,850 7 7 
Due after ten years2,631,945 2,442,103 2,691 3,450 
Total securities$3,869,048 $3,638,080 $2,700 $3,459 

The following table presents, as of March 31, 2022, 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$304,762 $291,679 
To secure repurchase agreements589,782 568,495 
Other securities pledged 243,779 232,466 
Total pledged securities$1,138,323 $1,092,640