5.    Goodwill and Intangible Assets

As of September 30, 2021 and December 31, 2020, the carrying value of goodwill was $244.7 million and $227.3 million, respectively. The total change in the carrying value of goodwill was primarily related to $17.1 million from the Meetrics acquisition. The remaining change in goodwill was deemed immaterial.

The following table summarizes the Company’s intangible assets and related accumulated amortization:

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Amortization expense for the three months ended September 30, 2021 and September 30, 2020 is $4.6 million and $4.4 million, respectively. Amortization expense related to intangible assets amounted to $13.5 million and $13.4 million for the nine months ended September 30, 2021 and September 30, 2020, respectively.

Estimated future expected amortization expense of intangible assets as of September 30, 2021 is as follows:

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The weighted-average remaining useful life by major asset classes as of September 30, 2021 is as follows:

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There were no impairments identified during the nine months ended September 30, 2021 or September 30, 2020.