Note 2 — Securities

The following is a summary of securities available for sale as of the dates indicated:

The amortized cost and estimated fair value of securities as of June 30, 2021 and December 31, 2020, by contractual or expected maturity, are shown below. Collateralized mortgage obligations are included in the table shown below based on their expected maturities. All other securities are included based on their contractual maturities.

The following table summarizes debt securities available-for-sale in an unrealized loss position for which an allowance for credit losses has not been recorded at June 30, 2021 and December 31, 2020, aggregated by major security type and length of time in a continuous unrealized loss position:

The Company evaluates its available-for-sale securities portfolio for impairment on a quarterly basis. This assessment takes into account the changes in the credit quality of these debt securities since acquisition and the likelihood of a credit loss occurring over the life of the securities.  In the event that a credit loss is expected to occur in the future, an allowance is established and a corresponding credit loss is recognized.  Based on this analysis, as of June 30, 2021, the Company determined that no credit losses are expected to be realized on the tax-exempt municipal bond portfolio.  The remainder of the portfolio consists of U.S. Treasury obligations, U.S. government agency securities, and U.S. government sponsored agency securities, all of which have the backing of the U.S. government, and are therefore not expected to incur credit losses.

Realized gains and losses on sales of securities and proceeds from sales of securities were as follows for the periods indicated:

There were no sales of securities during the three months ended June 30, 2021. During the three months ended June 30, 2020, there were $15.7 million in gains resulting from the sale of $495.6 million of securities previously recorded with $15.3 million in accumulated other comprehensive income.

During the six months ended June 30, 2021, there were $99,000 in gains resulting from the sale of $8.0 million of securities previously recorded with $142,000 unrealized gains in accumulated other comprehensive income. During the six months ended June 30, 2020, there were $15.7 million in net gains in earnings resulting from the sale of $495.6 million of securities previously recorded with $15.3 million unrealized gains in accumulated other comprehensive income.

Securities available for sale with market values of $40.5 million and $27.3 million as of June 30, 2021 and December 31, 2020, respectively, were pledged to secure borrowings from the Federal Reserve Bank (“FRB”) Discount Window.

At June 30, 2021, there were no holdings of securities of any one issuer, other than the U.S. government and its agencies in an amount greater than 10 percent of shareholders’ equity.