The following table summarizes the carrying amount of goodwill resulting from the acquisitions of 3VR, Thursby, and Viscount (in thousands):

	Premises			Identity		Total
Balance at December 31, 2018	$	5,796		$	3,490	$	9,286
Acquisition of business		1,173			—		1,173
Cancellation of holdback shares in connection with acquisition		(340	)		—		(340	)
Currency translation adjustment		19			—		19
Balance at June 30, 2019	$	6,648		$	3,490	$	10,138

In accordance with ASC 350, the Company tests goodwill for impairment on an annual basis, in the fourth quarter, or whenever events or changes in circumstances indicate that the carrying amount of these assets may not be recoverable. In testing for goodwill impairment, the Company compares the fair value of its reporting unit to its carrying value including the goodwill of that unit. If the carrying value, including goodwill, exceeds the reporting unit’s fair value, the Company will recognize an impairment loss for the amount by which the carrying amount exceeds the reporting unit’s fair value. During the six months ended June 30, 2019, the Company noted no indicators of goodwill impairment and concluded no further testing was necessary.

The following table summarizes the gross carrying amount and accumulated amortization for intangible assets resulting from acquisitions (in thousands):

				Developed			Customer
	Trademarks			Technology			Relationships			Total
Amortization period (in years)	5			10 – 12			4 – 12

Gross carrying amount at December 31, 2018	$	600		$	8,300		$	15,039		$	23,939
Accumulated amortization		(77	)		(3,978	)		(8,904	)		(12,959	)
Intangible assets, net at December 31, 2018	$	523		$	4,322		$	6,135		$	10,980

Gross carrying amount at June 30, 2019	$	762		$	9,106		$	15,754		$	25,622
Accumulated amortization		(152	)		(4,426	)		(9,666	)		(14,244	)
Intangible assets, net at June 30, 2019	$	610		$	4,680		$	6,088		$	11,378

Each period, the Company evaluates the estimated remaining useful lives of its intangible assets and whether events or changes in circumstances warrant a revision to the remaining period of amortization. If a revision to the remaining period of amortization is warranted, amortization is prospectively adjusted over the remaining useful life of the intangible asset. Intangible assets subject to amortization are amortized over their useful lives as shown in the table above. The Company evaluates its amortizable intangible assets for impairment at the end of each reporting period. The Company did not identify any impairment indicators during the six months ended June 30, 2019.

The following table illustrates the amortization expense included in the condensed consolidated statements of operations for the three and six months ended June 30, 2019 and 2018, respectively (in thousands):

	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2019		2018		2019		2018
Cost of revenue	$	224	$	187	$	445	$	336
Selling and marketing		419		345		838		643
Total	$	643	$	532	$	1,283	$	979

The estimated annual future amortization expense for purchased intangible assets with definite lives as of June 30, 2019 was as follows (in thousands):