	Steel Mills		Steel Products		Raw Materials		Total
Balance at December 31, 2018	$	591,986	$	862,773	$	729,577	$	2,184,336
Translation		-		2,163		-		2,163
Balance at September 28, 2019	$	591,986	$	864,936	$	729,577	$	2,186,499

Nucor completed its most recent annual goodwill impairment testing during the fourth quarter of 2018 and concluded that as of such time there was no impairment of goodwill for any of its reporting units.

The assessment performed in 2018 used forward-looking projections and included significant expected improvements in the future cash flows of one of the Company’s reporting units, Rebar Fabrication. The fair value of this reporting unit exceeded its carrying value by approximately 8% in the most recent assessment. We expect the 2019 operating results of this reporting unit to improve as compared to 2018. If our assessment of the relevant facts and circumstances changes, or the actual performance of this reporting unit falls short of expected results, noncash impairment charges may be required. Total goodwill associated with the Rebar Fabrication reporting unit was $354.9 million as of September 28, 2019 ($353.0 million as of December 31, 2018). 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 $69.4 million as of September 28, 2019 ($76.7 million as of December 31, 2018). 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 Company has monitored one of its reporting units, Grating, for potential triggering events since the impairment assessment performed in the fourth quarter of 2018. Due to lower than expected operating performance and anticipated changes to the reporting unit’s business strategy and structure, the Company determined a triggering event occurred in the third quarter of 2019 and performed an impairment assessment. The fair value of the Grating reporting unit exceeded its carrying value by approximately 17% 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, noncash impairment charges may be required. An impairment of goodwill may also lead us to record an impairment of other intangible assets. As of September 28, 2019, total goodwill and finite-lived intangible assets associated with the Grating reporting unit were $36.7 million and $3.3 million, respectively.

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 September 28, 2019 and December 31, 2018 (in thousands):

	September 28, 2019				December 31, 2018
	Gross Amount		Accumulated Amortization		Gross Amount		Accumulated Amortization
Customer relationships	$	1,421,107	$	767,027	$	1,418,250	$	713,656
Trademarks and trade names		177,620		95,450		176,046		87,680
Other		63,807		35,790		67,820		32,276
	$	1,662,534	$	898,267	$	1,662,116	$	833,612

Intangible asset amortization expense in the third quarter of 2019 and 2018 was $22.0 million and $22.1 million, respectively, and was $64.7 million and $66.7 million in the first nine months of 2019 and 2018, respectively. Annual amortization expense is estimated to be $87.1 million in 2019; $84.7 million in 2020; $83.5 million in 2021; $81.2 million in 2022; and $80.0 million in 2023.