6. Intangible Assets

Capitalized internal-use software

As of September 30, 2018 and December 31, 2017, capitalized internal-use software consisted of the following:

(in thousands)	September 30, 2018			December 31, 2017
Gross capitalized internal-use software	$	105,352		$	95,607
Less: Accumulated amortization		(70,463	)		(54,114	)
Capitalized internal use software, net	$	34,889		$	41,493

Amortization expense related to capitalized internal-use software was $5.0 million and $16.3 million during the three and nine months ended September 30, 2018, respectively, and $5.4 million and $15.6 million during the three and nine months ended September 30, 2017, respectively, and is included in cost of revenue in the consolidated statements of operations.

Acquisition intangible assets, net

As of September 30, 2018 and December 31, 2017, intangible assets, net consisted of the following:

	September 30, 2018							December 31, 2017
(in thousands)	Gross Carrying Amount		Accumulated Amortization			Net Carrying Amount		Gross Carrying Amount		Accumulated Amortization			Net Carrying Amount
Customer relationships	$	47,696	$	(41,981	)	$	5,715	$	47,696	$	(40,696	)	$	7,000
Trade name		6,072		(5,680	)		392		6,072		(5,192	)		880
Technology		22,007		(17,757	)		4,250		22,007		(16,293	)		5,714
Acquisition intangible assets, net	$	75,775	$	(65,418	)	$	10,357	$	75,775	$	(62,181	)	$	13,594

Amortization expense was $1.1 million and $3.2 million during the three and nine months ended September 30, 2018, respectively, and $1.1 million and $3.4 million during the three and nine months ended September 30, 2017, respectively.

Future amortization expense

As of September 30, 2018, future amortization expense by year is expected to be as follows (in thousands):

(in thousands)	Capitalized internal-use software, net		Acquisition intangible assets, net
Remainder of 2018	$	4,915	$	1,033
2019		11,243		3,839
2020		4,444		2,952
2021		70		1,705
2022		—		828
Total amortization expense	$	20,672	$	10,357

Future capitalized internal-use software amortization excludes $14.2 million of costs which are currently in the development phase.

Goodwill, net

The changes in the carrying amount of goodwill, net were as follows (in thousands):

Balance as of December 31, 2017		$	336,861
Balance as of September 30, 2018		$	336,861