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Investment Securities
 9 Months Ended
Sep. 30, 2021
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 September 30, 2021 and December 31, 2020: 
September 30, 2021
 (in thousands) Amortized CostUnrealized GainsUnrealized LossesFair Value
Available for sale:    
U.S. Treasury and agencies$806,469 $35,304 $(2,796)$838,977 
Obligations of states and political subdivisions272,275 11,851 (717)283,409 
Residential mortgage-backed securities and collateralized mortgage obligations
2,606,906 29,272 (35,393)2,600,785 
Total available for sale securities$3,685,650 $76,427 $(38,906)$3,723,171 
Held to maturity:    
Residential mortgage-backed securities and collateralized mortgage obligations
$2,795 $786 $— $3,581 
Total held to maturity securities$2,795 $786 $— $3,581 

December 31, 2020
 (in thousands) 
Amortized CostUnrealized GainsUnrealized LossesFair Value
Available for sale:    
U.S. Treasury and agencies$698,243 $64,271 $(312)$762,202 
Obligations of states and political subdivisions263,546 15,996 (31)279,511 
Residential mortgage-backed securities and collateralized mortgage obligations
1,839,711 51,583 (449)1,890,845 
Total available for sale securities$2,801,500 $131,850 $(792)$2,932,558 
Held to maturity:    
Residential mortgage-backed securities and collateralized mortgage obligations
$3,034 $849 $— $3,883 
Total held to maturity securities$3,034 $849 $— $3,883 

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 $12.2 million and $8.9 million as of September 30, 2021 and December 31, 2020, respectively, and is included in Other Assets.
Debt securities that were in an unrealized loss position as of September 30, 2021 and December 31, 2020 are presented in the following tables, based on the length of time individual securities have been in an unrealized loss position.

September 30, 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$119,093 $1,773 $18,805 $1,023 $137,898 $2,796 
Obligations of states and political subdivisions
53,099 717 — — 53,099 717 
Residential mortgage-backed securities and collateralized mortgage obligations
1,500,804 34,346 23,561 1,047 1,524,365 35,393 
Total temporarily impaired securities$1,672,996 $36,836 $42,366 $2,070 $1,715,362 $38,906 

December 31, 2020
Less than 12 Months12 Months or LongerTotal
  (in thousands)
Fair ValueUnrealized LossesFair ValueUnrealized LossesFair ValueUnrealized Losses
Available for sale:      
U.S. Treasury and agencies$29,493 $312 $— $— $29,493 $312 
Obligations of states and political subdivisions
4,357 31 — — 4,357 31 
Residential mortgage-backed securities and collateralized mortgage obligations
215,165 449 — — 215,165 449 
Total temporarily impaired securities$249,015 $792 $— $— $249,015 $792 

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 residential mortgage-backed securities and collateralized mortgage obligations portfolio in an unrealized loss position at September 30, 2021 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, 2021.

The following table presents the contractual maturities of debt securities at September 30, 2021:  

Available For SaleHeld To Maturity
  (in thousands) 
Amortized CostFair ValueAmortized CostFair Value
Due within one year$16,913 $17,150 $— $— 
Due after one year through five years122,809 128,548 2 2 
Due after five years through ten years967,343 1,004,037 8 8 
Due after ten years2,578,585 2,573,436 2,785 3,571 
Total securities$3,685,650 $3,723,171 $2,795 $3,581 

The following table presents, as of September 30, 2021, investment securities that 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$293,931 $301,370 
Other securities pledged principally to secure repurchase agreements746,122 770,020 
Total pledged securities$1,040,053 $1,071,390