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 of 3 to 20 years. Intangible assets that are fully amortized are removed from the disclosures.

Identified intangible assets by major classification, affected by the fluctuation of foreign currency rates, consist of the following (in millions):

			Accumulated			Net Book
	Gross		Amortization			Value
December 31, 2019:
Trademarks and patents	$	38	$	(9	)	$	29
Customer relationships		116		(57	)		59
Other		2		—			2
Total identified intangibles	$	156	$	(66	)	$	90

December 31, 2018:
Trademarks and patents	$	103	$	(31	)	$	72
Customer relationships		118		(46	)		72
Other		11		(11	)		—
Total identified intangibles	$	232	$	(88	)	$	144

During the fourth quarter of 2019, the Company made strategic decisions to discontinue the use of certain acquired trade names in order to eliminate branding dilution in the market and align the Company’s marketing around its DNOW brands. As of December 31, 2019, the Company completed the disposals of these trade names by abandonment and recognized impairment charges of $34 million in the U.S. reporting segment and $4 million in the International reporting segment. Such impairment was included in impairment charges in the consolidated statements of operations, representing the remaining carrying values of these specifically acquired intangible assets. The Company did not record impairment for intangible assets for the years ended December 31, 2018 and 2017.

Amortization expense was $19 million, $20 million and $22 million for the years ended December 31, 2019, 2018, and 2017, respectively. The following table represents the total estimated amortization of intangible assets for the five succeeding years, excluding assets held-for-sale (in millions):