4. Intangible Assets

Intangible assets consisted of the following (in thousands):


	December 31, 2017
	Gross carrying		Accumulated		Net carrying		Weighted average
	amount		amortization		amount		useful life (years)
Customer relationships	$	2,853	$	(1,628)	$	1,225	4.3
Artist relationships		12,235		(10,659)		1,576	4.1
Media content		91,445		(90,187)		1,258	5.1
Technology		6,204		(4,751)		1,453	4.6
Non-compete agreements		25		(20)		5	3.0
Trade names		9,889		(4,975)		4,914	10.2
	$	122,651	$	(112,220)	$	10,431


	December 31, 2016
	Gross carrying		Accumulated		Net carrying		Weighted average
	amount		amortization		amount		useful life (years)
Customer relationships	$	1,453	$	(1,252)	$	201	3.7
Artist relationships		11,916		(10,337)		1,579	4.1
Media content		91,616		(87,604)		4,012	5.1
Technology		5,654		(3,649)		2,005	4.7
Non-compete agreements		25		(11)		14	3.0
Trade names		7,539		(4,077)		3,462	10.2
	$	118,203	$	(106,930)	$	11,273

Identifiable finite-lived intangible assets are amortized on a straight-line basis over their estimated useful lives.

Amortization expense by classification is shown below (in thousands):


	Year ended December 31,
	2017		2016		2015
Service costs	$	3,212	$	7,037	$	12,680
Sales and marketing		692		2,319		3,862
Product development		1,101		987		1,440
General and administrative		723		557		724
Total amortization	$	5,728	$	10,900	$	18,706

Service costs for the years ended December 31, 2017, 2016 and 2015 include an accelerated amortization charge of $0.7 million, $1.9 million and $3.4 million, respectively, as a result of the removing certain content assets from service.

Based upon the current amount of intangible assets subject to amortization, the estimated amortization expense for the next five years as of December 31, 2017 is as follows (in thousands):


	Estimated
	Amortization
Year ending December 31,
2018	$	3,397
2019	$	1,959
2020	$	1,336
2021	$	1,172
2022	$	930
Thereafter	$	1,637