The change in the net carrying amount of goodwill for the three months ended April 3, 2021 by segment was as follows (in thousands):

	Steel Mills		Steel Products		Raw Materials		Total
Balance at December 31, 2020	$	612,470	$	887,625	$	729,577	$	2,229,672
Acquisitions		1,030		-		-		1,030
Translation		-		4,481		-		4,481
Balance at April 3, 2021	$	613,500	$	892,106	$	729,577	$	2,235,183

Nucor completed its most recent annual goodwill impairment testing during the fourth quarter of 2020 and concluded that as of such time 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 in 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 was profitable in 2020 and we expect it 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 was $368.2 million as of April 3, 2021 ($364.3 million as of December 31, 2020). 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 were $56.8 million as of April 3, 2021 ($58.8 million as of December 31, 2020). There have been no triggering events requiring an interim assessment for impairment of the Rebar Fabrication reporting unit since the most recent annual goodwill impairment testing date.

The annual assessment performed in 2020 for one of the Company’s reporting units, Grating, used forward-looking projections and included continued positive future cash flows. The fair value of this 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. Total goodwill associated with the Grating reporting unit was $37.2 million as of April 3, 2021 ($37.0 million as of December 31, 2020).

Intangible assets with estimated useful lives of five to 22 years are amortized on a straight-line or accelerated basis and were comprised of the following as of April 3, 2021 and December 31, 2020 (in thousands):

	April 3, 2021				December 31, 2020
	Gross Amount		Accumulated Amortization		Gross Amount		Accumulated Amortization
Customer relationships	$	1,422,453	$	856,406	$	1,421,962	$	838,443
Trademarks and trade names		162,462		101,991		162,365		100,000
Other		63,822		42,861		63,822		41,685
	$	1,648,737	$	1,001,258	$	1,648,149	$	980,128

Intangible asset amortization expense in the first quarter of 2021 and 2020 was $21.1 million and $21.5 million, respectively. 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.