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

	Premises		Identity		Total
Balance at December 31, 2017	$	—	$	—	$	—
Acquisition of businesses		5,796		3,490		9,286
Balance at December 31, 2018	$	5,796	$	3,490	$	9,286

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 quarter ended December 31, 2018, the Company noted no indicators of goodwill impairment and concluded no further testing 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, 2017	$	—		$	4,600		$	10,639		$	15,239
Accumulated amortization	0				(3,257	)		(7,617	)		(10,874	)
Intangible assets, net at December 31, 2017	$	—		$	1,343		$	3,022		$	4,365

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

Each period, the Company evaluates the estimated remaining useful lives of purchased 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 on a straight-line basis over their useful lives as outlined in the table above. The Company performs an evaluation of its amortizable intangible assets for impairment at the end of each reporting period. The Company did not identify any impairment indicators during the years ended December 31, 2018 and 2017.

The following table illustrates the amortization expense included in the consolidated statements of operations for the years ended December 31, 2018 and 2017 (in thousands):

	Year Ended December 31,
	2018		2017
Cost of revenue	$	721	$	448
Selling and marketing		1,364		1,007
Total	$	2,085	$	1,455

The estimated annual future amortization expense for purchased intangible assets with definite lives over the next five years is as follows (in thousands):