The change in the net carrying amount of goodwill for the years ended December 31, 2020 and 2019 by segment is as follows:

	(in thousands)
	Steel		Steel			Raw
	Mills		Products			Materials		Total
Balance, December 31, 2018	$	591,986	$	862,773		$	729,577	$	2,184,336
Acquisitions		—		12,623			—		12,623
Translation		—		4,104			—		4,104
Balance, December 31, 2019		591,986		879,500			729,577		2,201,063
Acquisitions		20,484		(821	)		—		19,663
Translation		—		8,946			—		8,946
Balance, December 31, 2020	$	612,470	$	887,625		$	729,577	$	2,229,672

Intangible assets with estimated useful lives of five to 22 years are amortized on a straight-line or accelerated basis and are comprised of the following:

	(in thousands)
	December 31, 2020				December 31, 2019
	Gross		Accumulated		Gross		Accumulated
	Amount		Amortization		Amount		Amortization
Customer relationships	$	1,421,962	$	838,443	$	1,412,954	$	767,532
Trademarks and trade names		162,365		100,000		162,183		92,258
Other		63,822		41,685		63,807		36,968
	$	1,648,149	$	980,128	$	1,638,944	$	896,758

Intangible asset amortization expense was $83.4 million in 2020 ($85.7 million in 2019 and $88.8 million in 2018). Annual amortization expense is estimated to be $82.7 million in 2021, $81.1 million in 2022, $80.4 million in 2023, $79.6 million in 2024 and $78.6 million in 2025.

The Company completed its annual goodwill impairment testing as of the first day of the fourth quarter of each of 2020, 2019 and 2018 and concluded that as of each such date there was no impairment of goodwill for any of its reporting units.

The annual assessment performed in 2020 for one of the Company’s reporting units, Rebar Fabrication, used forward-looking projections and included continued positive future cash flows. The fair value of this reporting unit exceeded its carrying value by approximately 99% in the most recent assessment. The reporting unit’s profitability in 2020 significantly increased from 2019, and we currently expect the reporting unit to be profitable in 2021. If our assessment of the relevant facts and circumstances changes, or the actual performance of this reporting unit falls short of expected results, non-cash impairment charges may be required. Total goodwill associated with the Rebar Fabrication reporting unit as of December 31, 2020 was $364.3 million. An impairment of goodwill may also lead us to record an impairment of other intangible assets. Total finite-lived intangible assets associated with the Rebar Fabrication reporting unit as of December 31, 2020 was $58.8 million.

The Company has continued to monitor one of its reporting units, Grating, for potential triggering events since the impairment assessment performed in the third quarter of 2019. No triggering events occurred, so the Company completed its annual goodwill impairment testing as of the first day of the fourth quarter of 2020. The fair value of the Grating reporting unit exceeded its carrying value by approximately 88% in the most recent assessment. If our assessment of the relevant facts and circumstances changes, or the actual performance of this reporting unit falls short of expected results, non-cash impairment charges may be required. As of December 31, 2020, total goodwill associated with the Grating reporting unit was $37.0 million.

There are no significant historical accumulated impairment charges, by segment or in the aggregate, related to goodwill.