causes an adverse effect on the fair value of our investment occurs, as evidenced by the presence of an impairment indicator, the fair value of our investment should be estimated. In 2016, ZIM experienced significant deterioration of its financial results, reported significant operating losses, negative equity and negative working capital mainly as a result of the adverse change in the general containership market conditions. As a result of these adverse conditions, we estimated the fair value of our equity investment in ZIM at nil, therefore we recorded an impairment loss amounting to $28.7 million in the year ended December 31, 2016. With regard to our debt securities in ZIM and HMM, we originally recognized these securities as held to maturity based on our positive intent and ability to hold these securities to maturity. These securities were initially recognized at amortized costs, net of other than temporary impairment losses. We evaluate these securities for other than temporary impairment at each reporting date. Debt securities are considered impaired if the fair value of the investment is less than its amortized costs. In our evaluation we consider the following (i) if we intend to sell these debt securities, (ii) it is more likely than not that we will be required to sell these securities before the recovery of their entire amortized cost basis or (iii) if a credit loss exists, which means that we do not expect to recover the entire amortized cost basis of these securities. With regard to ZIM debt securities, as a result of the deterioration of ZIM’s financial results in 2016, as described above, we do not expect the present value of future cash flows to be collected to exceed their amortized cost basis due to a change in the timing of these expected cash flows. Thus an other than temporary impairment, a credit loss, has occurred as of December 31, 2016 amounting to $0.7 million. On March 28, 2017, we sold $13.0 million principal amount of HMM notes carried at amortized costs of $8.6 million for gross cash proceeds on sale of $6.2 million, which were used to repay related outstanding debt obligations. The loss on sale of $2.4 million was recognized under ‘‘Other income/ (expenses), net’’ in the Consolidated Statements of Operations for the year ended December 31, 2017. The sale of these notes resulted in a transfer of all remaining held to maturity HMM notes and ZIM notes into the available for sale securities at fair value and unrealized losses amounting to $38.2 million and $36.4 million as of December 31, 2019 and 2018, respectively, were recognized in other comprehensive loss. As of December 31, 2019, we do not intend to sell these debt securities and we evaluate that it is not more likely than not that we will be required to sell these debt securities before the recovery of their amortized cost basis. No other than temporary impairment loss was identified with regard to HMM and ZIM debt securities as of December 31, 2019. Impairment of Vessels We evaluate the net carrying value of our vessels for possible impairment when events or conditions exist that cause us to question whether the carrying value of the vessels will be recovered from future undiscounted net cash flows. An impairment charge would be recognized in a period if the fair value of the vessels was less than their carrying value and the carrying value was not recoverable from future undiscounted cash flows. Considerations in making such an impairment evaluation would include comparison of current carrying value to anticipated future operating cash flows, vessel market values, expectations with respect to future operations, and other relevant factors. As of December 31, 2019, we concluded that events occurred and circumstances had changed, which may trigger the existence of potential impairment of some of our vessels. These indicators included volatility in the charter market and the vessels’ market values, as well as the potential impact the current marketplace may have on our future operations. As a result, we performed an impairment assessment of certain of our vessels by comparing the undiscounted projected net operating cash flows for each vessel to their carrying value. Our strategy is to charter our vessels under multi-year, fixed rate period charters that range from less than one to 18 years for our current vessels, providing us with contracted stable cash flows. The significant factors and assumptions we used in our undiscounted
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