6.Goodwill and Other Intangible Assets

Goodwill

Changes in the carrying amount of goodwill were as follows:

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Our goodwill is not deductible for federal income tax purposes. See Note. 4. Acquisitions.

Intangible Assets

All our intangible assets are subjected to amortization and are related to customer contracts acquired through acquisitions. The components of the intangible assets are as follows:

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Substantially all the net carrying value of our intangible assets is attributable to contracts that expire in 2035. The table below shows the expected amortization of intangibles as of December 31, 2021:

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Amortization expense was $32.6 million, $20.4 million and $6.1 million for the years ended December 31, 2021, 2020 and 2019, respectively.