Valuation of Goodwill
Goodwill balances by reporting unit are as follows:
Goodwill represents the purchase price paid in excess of the fair value of net tangible and intangible assets acquired. Goodwill is not amortized but is subject to an impairment test at a reporting unit level on an annual basis and when circumstances indicate that an impairment is more-likely-than-not. Such circumstances that might indicate an impairment is more-likely-than-not include a significant adverse change in the business climate for one of our reporting units or a decision to dispose of a reporting unit or a significant portion of a reporting unit. The test for goodwill impairment is a two-step process. The first step of the test is performed by comparing the fair value of each reporting unit to its carrying amount, including recorded goodwill. If the carrying amount of a reporting unit exceeds its fair value, the second step is performed to measure the amount of the impairment, if any, by comparing the implied fair value of goodwill to its carrying amount. Any resulting impairment would be recorded in the current period.
Determining the fair value of a reporting unit for purposes of the goodwill impairment test or for changes in our operating structure is judgmental in nature and involves the use of estimates and assumptions. These estimates and assumptions could have a significant impact on whether or not an impairment charge is recognized and also the magnitude of any such charge. Estimates of fair value are primarily determined using discounted cash flows and market multiples from publicly traded comparable companies. These approaches use significant estimates and assumptions including projected future cash flows, discount rate reflecting the inherent risk in future cash flows, perpetual growth rate and determination of appropriate market comparables.
We evaluate our reporting units when changes in our operating structure occur, and if necessary, reassign goodwill using a relative fair value allocation approach. As described in Note 18 to our Consolidated Financial Statements in Item 8 of this Form 10-K, beginning on October 1, 2017, we concluded that CMS became a separate operating segment. In conjunction with the changes to reporting units, we reassigned goodwill between CGD and CMS based on their relative fair values as of October 1, 2017. We estimated the fair value of CGD and CMS at October 1, 2017 based upon market multiples from publicly traded comparable companies in addition to discounted cash flows models for CMS and for a combination of CGD and CMS based on discrete financial forecasts developed by management for planning purposes. Cash flows beyond the discrete forecasts were estimated based on projected growth rates and financial ratios, influenced by an analysis of historical ratios and by calculating a terminal value at the end of the discrete financial forecasts. For the October 1, 2017 valuations, future cash flows were discounted to present value using a discount rate of 13% for our CMS reporting unit and 11% for the combination of our CGD and CMS reporting units.
For the first step of our fiscal 2019 annual impairment test, we estimated the fair value of CTS based upon market multiples from publicly traded comparable companies and for CGD and CMS, we estimated the fair value based upon a combination of market multiples from publicly traded comparable companies and discounted cash flow models. The discounted cash flows were based on discrete three-year financial forecasts developed by management for planning purposes. Cash flows beyond the discrete forecasts were estimated based on projected growth rates and financial ratios, influenced by an analysis of historical ratios and by calculating a terminal value at the end of the three-year forecasts. The future cash flows were discounted to present value using a discount rate of 15% for CGD and 12.5% for CMS. The results of our 2019 annual impairment test indicated that the estimated fair value for our CTS and CGD reporting units exceeded their carrying amounts by over 100%, while the estimated fair value of our CMS reporting unit exceeded its carrying amount by over 60%.
Unforeseen negative changes in future business or other market conditions for any of our reporting units including margin compression or loss of business, could cause recorded goodwill to be impaired in the future. Also, changes in estimates and assumptions we make in conducting our goodwill assessment could affect the estimated fair value of our reporting units and could result in a goodwill impairment charge in a future period.
Table:
September 30, | 2019 | 2018 | 2017
 |  | (in millions) | 
Cubic Transportation Systems | $  254.6 | $  49.8 | $  50.9
Cubic Mission Solutions | 181.4 | 138.1 | —
Cubic Global Defense | 142.1 | 145.7 | 270.7
Total goodwill | $  578.1 | $  333.6 | $  321.6


Question: In which year was the goodwill balance for  Cubic Transportation Systems the lowest?
Answer: 2018
Calculation: 49.8<50.9<254.6