Changes in the carrying amount of Skyhook goodwill is as follows (amounts in thousands):

Skyhook’s legacy U-TDOA service was historically largely dependent on one wireless carrier (AT&T), which accounted for approximately 80% - 90% of Skyhook’s overall revenue related to its U-TDOA service. During September 2015, AT&T gave notice that it did not intend to renew its contract, which expired on December 31, 2015. The Company believed that the receipt of the notification represented a significant change in circumstances since we last performed our annual goodwill impairment test. Accordingly, we performed a goodwill impairment test upon receipt of the notification. At that time, the estimated fair value of the reporting unit was primarily determined based on the cash and cash equivalents held by the reporting unit, and when compared to its carrying value, it was concluded that a goodwill impairment did not exist. The carrying value of Skyhook included a $35.5 million deferred revenue liability related to the contract with AT&T. Upon expiration of the contract on December 31, 2015, the deferred revenue was recognized, as all contractual obligations were satisfied at that time. The recognition of this deferred revenue liability increased the reporting unit carrying value. As a result, the Company determined the fair value of Skyhook. As the reporting unit’s carrying value exceeded the fair value, we performed a Step 2 impairment test. The fair value of Skyhook, including the related intangibles and goodwill, was determined using Skyhook’s projections of future operating performance and applying a combination of market multiples (market approach) and discounted cash flow (income approach) calculations (Level 3). The impairment test resulted in a $20.7 million impairment loss related to Skyhook’s goodwill related to Skyhook’s legacy U-TDOA service during December 2015.

The estimated future amortization expense for the next five years related to intangible assets with definite lives as of December 31, 2016 is as follows (amounts in thousands):