4. Cash Equivalents and Marketable Securities

Cash equivalents and marketable securities consisted of the following:

As of June 30, 2019, $15.1 million of marketable securities had maturities of more than one year and are classified as long-term assets.  As of December 31, 2018, no marketable securities had maturities of more than one year.

There were $13.0 million and $52.6 million investments in an unrealized loss position as of June 30, 2019 and December 31, 2018, respectively. During the three and six months ended June 30, 2019 and 2018 we did not record any other-than-temporary impairment charges on our available-for-sale securities. Based upon our quarterly impairment review, we determined that the unrealized losses were not attributed to credit risk, but were primarily associated with changes in interest rates. Based on the scheduled maturities of our investments and our determination that it was more likely than not that we will hold these investments for a period of time sufficient for a recovery of our cost basis, we concluded that the unrealized losses in our investment securities were not other-than-temporary.