and third-party finance and insurance products), (C) have similar target markets and customers (generally individuals), and (D) have similar distribution and marketing practices (all distribute products and services through dealership facilities that market to customers in similar fashions). The geographic reporting units are Eastern, Central, and Western United States and International. Our Retail Commercial Truck reportable segment has been determined to represent one operating segment and reporting unit. The goodwill included in our Other reportable segment relates primarily to our commercial vehicle distribution operating segment. There is no goodwill recorded in our Non-Automotive Investments reportable segment.
For our Retail Automotive and Retail Commercial Truck reporting units, we prepared a qualitative assessment of the carrying value of goodwill using the criteria in ASC 350-20-35-3 to determine whether it is more likely than not that a reporting unit’s fair value is less than its carrying value. If it were determined through the qualitative assessment that a reporting unit’s fair value is more likely than not greater than its carrying value, additional analysis would be unnecessary. During 2016, we concluded that it was not more likely than not that any of the retail automotive or retail commercial truck reporting units’ fair values were less than their carrying amount. If additional impairment testing was necessary, we would have estimated the fair value of our reporting units using an “income” valuation approach. The “income” valuation approach estimates our enterprise value using a net present value model, which discounts projected free cash flows of our business using the weighted average cost of capital as the discount rate. In connection with this process, we also reconcile the estimated aggregate fair values of our reporting units to our market capitalization. We believe this reconciliation process is consistent with a market participant perspective. This consideration would also include a control premium that represents the estimated amount an investor would pay for our equity securities to obtain a controlling interest, and other significant assumptions including revenue and profitability growth, franchise profit margins, residual values and the cost of capital.
For our Other reportable segment, we performed our impairment test by comparing the estimated fair value of each reporting unit with its carrying value. We estimated the fair value of these reporting units using an “income” valuation approach, as described above. We concluded that the fair value of each of these reporting units exceeded its carrying value.
We account for each of our investments under the equity method, pursuant to which we record our proportionate share of the investee’s income each period. The net book value of our investments was $893.4 million and $336.4 million as of December 31, 2016 and 2015, respectively, including $823.8 million and $290.6 million relating to PTL as of December 31, 2016 and 2015, respectively. In July 2016, we increased our ownership interest in PTL from 9.0% to 23.4% as a result of our acquisition of an additional 14.4% ownership interest, as discussed previously.
Investments for which there is not a liquid, actively traded market are reviewed periodically by management for indicators of impairment. If an indicator of impairment is identified, management estimates the fair value of the investment using a discounted cash flow approach, which includes assumptions relating to revenue and profitability growth, profit margins and our cost of capital. Declines in investment values that are deemed to be other than temporary may result in an impairment charge reducing the investments’ carrying value to fair value.
We retain risk relating to certain of our general liability insurance, workers’ compensation insurance, vehicle physical damage insurance, property insurance, employment practices liability insurance, directors and officers insurance and employee medical benefits in the U.S. As a result, we are likely to be responsible for a significant portion of the claims and losses incurred under these programs. The amount of risk we retain varies by program, and, for certain exposures, we have pre-determined maximum loss limits for certain individual claims and/or insurance periods. Losses, if any, above the pre-determined loss limits are paid by third-party insurance carriers. Certain insurers have limited available property coverage in response to the natural catastrophes experienced in recent years. Our estimate of future losses is prepared by management using our historical loss experience and industry-based development factors. Aggregate reserves relating to retained risk were $28.3 million and $26.4 million as of December 31, 2016 and 2015, respectively.