Intangible Assets

The following tables summarize the Company’s total purchased intangible assets (in thousands):



	Gross		Accumulated Amortization			Net
December 31, 2017
Purchased technology	$	114,754	$	(110,674	)	$	4,080
Patents	6,472		(6,472		)	—
Customer lists	37,000		(37,000		)	—
Trade names	11,536		(8,754		)	2,782
Total intangible assets	$	169,762	$	(162,900	)	$	6,862
December 31, 2016
Purchased technology	$	114,754	$	(102,967	)	$	11,787
Patents	6,472		(6,472		)	—
Customer lists	37,000		(34,784		)	2,216
Trade names	11,536		(7,964		)	3,572
Backlog	$	2,127	$	(2,127	)	—
Total intangible assets	$	171,889	$	(154,314	)	$	17,575

Intangible assets amortization expense was $10.7 million, $12.7 million and $12.9 million for the years ended December 31, 2017, 2016 and 2015. The Company’s purchased intangible assets have remaining useful lives of one to four years as of December 31, 2017. The Company reviews for impairment all of its purchased intangible assets whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. During 2017, the Company wrote off the fully amortized intangible asset related to Backlog as this amount no longer provides a future benefit to the Company.

The estimated future amortization expense of purchased intangible assets as of December 31, 2017 is as follows (in thousands):



For the Years Ending December 31,	Estimated Intangible Amortization Amount
2018	$	4,870
2019	790
2020	790
2021	412
Thereafter	—
Total estimated future amortization expense	$	6,862