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Intangible Assets, Goodwill and Other
 12 Months Ended
Dec. 31, 2021
Goodwill and Intangible Assets Disclosure [Abstract]  
Goodwill and Intangible Assets Goodwill and Intangible Assets
Goodwill
There were no changes in the carrying value of goodwill during the years ended December 31, 2021 and 2020.
Intangibles
The following is a summary of the Company's intangible assets.
As of December 31, 2021As of December 31, 2020
Gross Carrying ValueAccumulated AmortizationNet Book ValueGross Carrying ValueAccumulated AmortizationNet Book Value
Site rental contracts and tenant relationships
$7,854 $(3,872)$3,982 $7,797 $(3,432)$4,365 
Other intangible assets143 (79)64 143 (75)68 
Total$7,997 $(3,951)$4,046 $7,940 $(3,507)$4,433 
Amortization expense related to intangible assets is classified as "Depreciation, amortization and accretion" on the Company's consolidated statement of operations and comprehensive income (loss) and was $444 million, $439 million, and $428 million for the years ended December 31, 2021, 2020 and 2019, respectively.
The estimated annual amortization expense related to intangible assets for the years ending December 31, 2022 to 2026 is as follows:
 Years Ending December 31,
20222023202420252026
Estimated annual amortization$447 $445 $396 $374 $370