that excess. The fair value is determined using a discounted cash flow approach, which includes assumptions about revenue and profitability growth, profit margins, and the cost of capital. We also evaluate in connection with the annual impairment testing whether events and circumstances continue to support our assessment that the other indefinite-lived intangible assets continue to have an indefinite life.
Goodwill impairment is assessed at the reporting unit level annually on October 1 and upon the occurrence of an indicator of impairment. Our operations are organized by management into operating segments by line of business and geography. We have determined that we have four reportable segments as defined in generally accepted accounting principles for segment reporting: (i) Retail Automotive, consisting of our retail automotive dealership operations; (ii) Retail Commercial Truck, consisting of our retail commercial truck dealership operations in the U.S. and Canada; (iii) Other, consisting of our commercial vehicle and power systems distribution operations and other non-automotive consolidated operations; and (iv) Non-Automotive Investments, consisting of our equity method investments in non-automotive operations. We have determined that the dealerships in each of our operating segments within the Retail Automotive reportable segment are components that are aggregated into six reporting units for the purpose of goodwill impairment testing, as they (A) have similar economic characteristics (all are automotive dealerships having similar margins), (B) offer similar products and services (all sell new and/or used vehicles, service, parts 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 reporting units are Eastern, Central, and Western United States, CarSense, International, and CarShop. 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 prepare 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. If additional impairment testing was necessary, we would estimate 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 reporting units, we perform our impairment test by comparing the estimated fair value of each reporting unit with its carrying value. We estimate the fair value of these reporting units using an “income” valuation approach, as described above.
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 $1,184.9 million and $893.4 million as of September 30, 2017 and December 31, 2016, respectively, including $1,109.5 million and $823.8 million relating to PTL as of September 30, 2017 and December 31, 2016, 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, and in September 2017, we acquired an additional 5.5% ownership interest, as discussed previously. We currently hold a 28.9% ownership interest in PTL.
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