4. Cash Equivalents and Marketable Securities

Cash equivalents are highly liquid investments that are readily convertible into cash with original maturities of three months or less when purchased. Marketable securities consist of securities with original maturities greater than 90 days when purchased. The Company classifies these marketable securities as available-for-sale and records them at fair value in the accompanying condensed consolidated balance sheets. Unrealized gains or losses are included in accumulated other comprehensive loss. Premiums or discounts from par value are amortized to investment income over the life of the underlying security.

Cash equivalents and marketable securities consisted of the following at June 30, 2018 and December 31, 2017 (in thousands):

Although available to be sold to meet operating needs or otherwise, securities are generally held through maturity. The cost of securities sold is determined based on the specific identification method for purposes of recording realized gains and losses. During the three and six months ended June 30, 2018 and 2017, there were no realized gains or losses on sales of investments, and no investments were adjusted for other-than-temporary declines in fair value.

At June 30, 2018, the Company held four securities that were in an unrealized loss position. The aggregate fair value of securities held by the Company in an unrealized loss position for less than 12 months as of June 30, 2018 was $10.0 million, and there were no securities held by the Company in an unrealized loss position for more than 12 months. The Company has the intent and ability to hold such securities until recovery. The Company determined that there was no material change in the credit risk of the above investments. As a result, the Company determined it did not hold any investments with an other-than-temporary impairment as of June 30, 2018.