b'payment networks.Interchange fees from cardholder transactions are primarily driven by a percentage of the underlying transaction value and are recognized daily, concurrently with the transaction processing services. Trust fee income:Income is primarily comprised of fees earned from the management and administration of trusts and estates, the sale of insurance products, and investment brokerage and insurance services.Fees that are transaction-based (e.g., execution of trades and insurance commissions) are recognized at the time of the transaction.Other fees are earned over time as the contracted monthly or quarterly services are provided and are generally assessed based on either account activity or the market value of assets under management at month end.Other operating income: Income is primarily comprised of revenues generated by life insurance policies as a result of increases in cash surrender values as premiums are paid, and by the redemption and payout of the policies. These revenues are recognized at the time of carriers reporting cash surrender values to the Bank and at the time proceeds are received. These revenues are outside the scope of ASU 2014-09. Also included in other operating income are safe deposit box fees which are recognized in income at the time or within the same period that the Banks performance obligation is satisfied.These revenues are within the scope of ASU 2014-09.Gains from sales and calls of securities: Gains are recognized at the time the investments are marked to market (generally daily) or upon being realized, as applicable. These revenues are outside the scope of ASU 2014-09.Gains on sale of assets: Realized gains on the sale of assets (including sales of loans, other real estate owned, disposal of premises, equipment, and software, or repossessed assets) are recognized at a point in time once control of the assets has transferred to the buyers and collectability of the transaction price is reasonably assured.NOTE 3.SECURITIES The amortized cost and estimated market value of debt securities at December 31, 2019 and 2018 by contractual maturity are shown below.Expected maturities will differ from contractual maturities because borrowers may have the right to call or prepay obligations with or without call or prepayment penalties.Securities as of December 31 are summarized as follows: 12'