During the year ended December 31, 2019, we purchased certain intangible assets and completed several business acquisitions that were not material to our consolidated financial statements, either individually or in the aggregate. Accordingly, pro forma historical results of operations related to these business acquisitions during the year ended December 31, 2019 have not been presented. We have included the financial results of these business acquisitions in our consolidated financial statements from their respective dates of acquisition.

Goodwill generated from all business acquisitions completed was primarily attributable to expected synergies from future growth and potential monetization opportunities. The amount of goodwill generated that was deductible for tax purposes was not material.

The changes in the carrying amount of goodwill for the years ended December 31, 2019 and 2018 are as follows (in millions):



Balance as of December 31, 2017	$	18,221
Goodwill acquired	88
Effect of currency translation adjustment	(8		)
Balance as of December 31, 2018	18,301
Goodwill acquired	408
Effect of currency translation adjustment	6
Balance as of December 31, 2019	$	18,715

The following table sets forth the major categories of the intangible assets and the weighted-average remaining useful lives for those assets that are not already fully amortized (in millions):



		December 31, 2019							December 31, 2018
	Weighted-Average Remaining Useful Lives (in years)	Gross Carrying Amount		Accumulated Amortization			Net Carrying Amount		Gross Carrying Amount		Accumulated Amortization			Net Carrying Amount
Acquired users	1.8	$	2,056	$	(1,550	)	$	506	$	2,056	$	(1,260	)	$	796
Acquired technology	2.6	1,158		(986		)	172		1,002		(871		)	131
Acquired patents	4.6	805		(625		)	180		805		(565		)	240
Trade names	2.0	635		(604		)	31		629		(517		)	112
Other	3.3	162		(157		)	5		162		(147		)	15
Total intangible assets		$	4,816	$	(3,922	)	$	894	$	4,654	$	(3,360	)	$	1,294

Amortization expense of intangible assets for the years ended December 31, 2019, 2018, and 2017 was $562 million, $640 million, and $692 million, respectively.

As of December 31, 2019, expected amortization expense for the unamortized acquired intangible assets for the next five years and thereafter is as follows (in millions):