8. Goodwill and Intangible Assets

Goodwill and intangible assets consisted of the following at December 31:


	2019						2018
	Gross				Net		Gross				Net
	Carrying		Accumulated		Carrying		Carrying		Accumulated		Carrying
(in thousands)	Amount		Amortization		Amount		Amount		Amortization		Amount
Goodwill
T&D	$	40,224	$	—	$	40,224	$	40,224	$	—	$	40,224
C&I		25,836		—		25,836		16,364		—		16,364
Total goodwill	$	66,060	$	—	$	66,060	$	56,588	$	—	$	56,588
Amortizable Intangible Assets
Backlog	$	5,289	$	4,039	$	1,250	$	2,789	$	1,889	$	900
Customer relationships		31,381		6,623		24,758		17,280		4,970		12,310
Trade names		695		218		477		695		172		523
Indefinite-lived Intangible Assets
Trade names		28,455		—		28,455		19,533		—		19,533
Total intangible assets	$	65,820	$	10,880	$	54,940	$	40,297	$	7,031	$	33,266

The increase in goodwill as of December 31, 2019 compared to December 31, 2018 was primarily due to the allocation of $9.5 million of goodwill related to the acquisition of CSI identified during the ongoing analysis of the purchase accounting. The increase in intangible assets also related to the acquisition of CSI and are being amortized on a straight-line basis over periods ranging up to 12 years. Additional financial information related to this acquisition is provided in Note 2 –Acquisitions to the Financial Statements. Immaterial foreign currency translation adjustments related to goodwill and intangible assets are netted with the amounts indicated above.

Customer relationships and backlog are being amortized on a straight-line method over an estimated useful life ranging up to 12.5 years and the remaining life of the contract, respectively, and have been determined to have no residual value. Amortizable trade names are being amortized on a straight-line basis over an estimated useful life of approximately 15 years. Certain trade names have indefinite lives and, therefore, are not being amortized. Intangible asset amortization expense was $3.8 million, $1.8 million and $0.5 million for the years ended December 31, 2019, 2018 and 2017, respectively.

As of December 31, 2019, estimated future intangible asset amortization expense for the each of the next five years and thereafter was as follows:


	Future
	Amortization
(in thousands)	Expense
2020	$	3,587
2021		2,312
2022		2,312
2023		2,312
2024		2,312
Thereafter		13,650
Total	$	26,485