6.Intangible Assets

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

​

​

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, 2022:

​

​

Amortization expense totaled $36.7 million, $32.6 million, and $20.4 million for the years ended December 31, 2022, 2021 and 2020, respectively.