Intangible assets other than goodwill at December 31, 2018 and December 31, 2017 are summarized as follows:



	2018							2017
(in thousands)	Gross		Accumulated Amortization			Net		Gross		Accumulated Amortization			Net		Useful Life (in years)	Weighted Average Useful Life Remaining (in years)
Intangible assets with finite lives:
Customer relationships	$	103,332	$	(67,129	)	$	36,203	$	105,505	$	(57,796	)	$	47,709	1-12	5
Acquired technology	52,691		(47,546		)	5,145		54,716		(39,644		)	15,072		1-2	2
Trade names	25,096		(17,669		)	7,427		25,813		(15,552		)	10,261		3-6	5
Patent costs	11,032		(8,382		)	2,650		17,909		(7,338		)	10,571		1-19	14
Trade secrets	19,374		(13,574		)	5,800		19,431		(11,530		)	7,901		2-4	3
Acquired patents	16,212		(13,160		)	3,052		16,661		(11,969		)	4,692		6-8	7
Other	26,551		(18,553		)	7,998		20,012		(17,435		)	2,577		1-2	1
Total intangible assets	$	254,288	$	(186,013	)	$	68,275	$	260,047	$	(161,264	)	$	98,783	1-19	5

The Company includes all amortization from assets attributable to the generation of revenue in the Cost of Sales line item in the Statement of Operations. Amortization related to assets that are not attributable to the generation of revenue are included in the Research and Development and Selling and General Administrative line items in the Statement of Operations. Amortization expense related to intangible assets was $29,722, $35,559 and $35,124 for the years ended December 31, 2018, 2017 and 2016, respectively. Amortization of these intangible assets is calculated on a straight-line basis.

Annual amortization expense for intangible assets is expected to be $20,411 in 2019, $17,308 in 2020, $12,859 in 2021, $7,493 in 2022 and $2,526 in 2023.

For discussion on intangible asset impairment testing, see Note 2.