Identified intangible assets with determinable lives consist primarily of customer relationships, trade names, trademarks and patents, and non-compete agreements acquired in acquisitions, and are being amortized on a straight-line basis over the estimated useful lives. Intangible assets that are fully amortized are removed from the disclosures. The acquisition date fair value of intangible assets relating to the acquisitions completed in December 2022 were preliminary. See Note 20 “Transactions” for additional information.

For the year ended December 31, 2021, the Company recognized $2 million of impairment in the U.S. reporting segment for certain customer relationship intangible assets due to decline in customer activities. During the first quarter of 2020, the results of the Company's test for impairment of goodwill and the other negative market indicators described in Note 7 “Goodwill” were a triggering event that indicated that its long-lived assets, including property, plant and equipment, operating ROU assets and intangible assets, were possibly impaired. As a result, for the quarter ended March 31, 2020 and the year ended December 31, 2020, the Company recognized $62 million and $22 million of impairment for intangible assets in the U.S. and International reporting segments, respectively. Such impairment was included in impairment and other charges in the consolidated statements of operations.

Identified intangible assets by major classification consist of the following (in millions):


			Accumulated			Net Book
	Gross		Amortization			Value
December 31, 2022:
Trade names and patents	$	4	$	—		$	4
Customer relationships		19		(2	)		17
Other		4		—			4
Total identified intangibles	$	27	$	(2	)	$	25

December 31, 2021:
Customer relationships	$	9	$	(1	)	$	8
Other		1		—			1
Total identified intangibles	$	10	$	(1	)	$	9

Amortization expense was $2 million, $2 million and $4 million for the years ended December 31, 2022, 2021, and 2020, respectively. The following table represents the total estimated amortization of intangible assets for the five succeeding years (in millions):


For the Year Ending December 31,	Estimated Amortization Expense
2023	$	4
2024		4
2025		3
2026		3
2027		3