GOODWILL AND OTHER INTANGIBLE ASSETS

The following table summarizes goodwill activity by segment for the nine months ended September 30, 2018 (in thousands):



	North America		International			Consolidated
Balance as of December 31, 2017	$	178,685	$	108,304		$	286,989
Goodwill related to acquisition	—		46,217			46,217
Foreign currency translation	—		(5,776		)	(5,776		)
Balance as of September 30, 2018	$	178,685	$	148,745		$	327,430

The following table summarizes intangible assets as of September 30, 2018 and December 31, 2017 (in thousands):



	September 30, 2018						December 31, 2017
Asset Category	Gross Carrying Value		Accumulated Amortization		Net Carrying Value		Gross Carrying Value		Accumulated Amortization		Net Carrying Value
Customer relationships	$	55,758	$	49,908	$	5,850	$	56,749	$	46,513	$	10,236
Merchant relationships	30,650		11,769		18,881		11,598		9,853		1,745
Trade names	14,390		11,298		3,092		12,077		10,469		1,608
Developed technology	37,093		36,680		413		36,864		36,864		—
Patents	36,184		16,236		19,948		19,031		15,204		3,827
Other intangible assets	11,294		10,446		848		10,875		9,095		1,780
Total	$	185,369	$	136,337	$	49,032	$	147,194	$	127,998	$	19,196

Amortization of intangible assets is computed using the straight-line method over their estimated useful lives, which range from 1 to 10 years. Amortization expense related to intangible assets was $3.9 million and $6.0 million for the three months ended September 30, 2018 and 2017, respectively, and $10.3 million and $17.6 million for the nine months ended September 30, 2018 and 2017, respectively. As of September 30, 2018, estimated future amortization expense related to intangible assets is as follows (in thousands):



Remaining amounts in 2018	$	4,448
2019	14,179
2020	8,279
2021	7,553
2022	7,237
Thereafter	7,336
Total	$	49,032

Sale of Intangible Assets

On September 15, 2017, we sold customer lists and other intangible assets in certain food delivery markets to a subsidiary of Grubhub Inc. ("Grubhub"). We recognized a pretax gain on the sale of assets of $17.1 million, which represents the excess of the $19.8 million in net proceeds received, consisting of $20.0 million in cash less $0.2 million in transaction costs, over the $2.7 million net book value of the assets upon closing of the transaction.