Acacia’s only identifiable intangible assets are patents and patent rights, with estimated remaining economic useful lives ranging from one to six years. For all periods presented, all of Acacia’s identifiable intangible assets were subject to amortization. The gross carrying amounts and accumulated amortization related to investments in intangible assets as of December 31, 2017 and 2016 are as follows (in thousands):



	2017			2016

Gross carrying amount - patents	$	444,137		$	444,362
Accumulated amortization - patents⁽¹⁾	(382,220		)	(358,043		)
Patents, net	$	61,917		$	86,319

⁽¹⁾ Includes patent impairment charges for the applicable periods.

The weighted-average remaining estimated economic useful life of Acacia’s patents and patent rights is 4 years. Scheduled annual aggregate amortization expense is estimated to be $20,542,000 in 2018, $18,527,000 in 2019, $6,134,000 in 2020, $5,261,000 in 2021, $5,256,000 in 2022 and $6,197,000 thereafter.

For the years ended December 31, 2017, 2016 and 2015, Acacia paid patent investment costs totaling $0, $1,225,000 and $19,504,000, respectively. The patents have initial estimated economic useful lives ranging from two to seven years.

Acacia recorded impairment of patent-related intangible asset charges totaling $2,248,000, $42,340,000 and $74,731,000 for the years ended December 31, 2017, 2016 and 2015, respectively. The impairment charges related to impairments of patent portfolios due to a reduction in expected estimated future net cash flows and certain patent portfolios that management determined it would no longer allocate future resources to in connection with the licensing and enforcement of such portfolios, due primarily to adverse litigation outcomes, potential prior art related complexities and/or the overall determination that future resources would be allocated to other licensing and enforcement programs with higher potential return profiles. The impairment charges for the periods presented consisted of the excess of the asset’s carrying value over its estimated fair value.

In December 2015, Acacia’s subsidiary Adaptix, Inc. received a jury verdict in its case against Alcatel Lucent USA, Inc., and others. The jury returned a verdict that the asserted claims of the patent at issue were invalid and non-infringed. The Adaptix trial loss resulted in a reduction in estimated cash flows for the Adaptix portfolio expected to be realized from future licensing and enforcement activities, leading to partial impairment charges on the portfolio in the fourth quarter of 2015. Fiscal year 2016 patent impairment charges included the impairment of the remaining carrying value for the Adapitx portfolio. In addition, for the year ended December 31, 2015 analysis, management considered the impact of the fourth quarter 2015 adverse trial outcomes on its estimates of future cash flows that could be realized from future licensing and enforcement activities for other patent portfolios. Estimates of future cash flows for these portfolios were reduced in part in connection with the Company’s assessment of probabilities of realization given the recent adverse trial outcomes. Additionally, patent impairment charges include the carrying value of other patent portfolios for which, in 2015, the Company experienced adverse litigation or trial outcomes, leading to a reduction in or elimination of expected future cash flows. In addition, headcount reductions and internal staff optimization efforts led to changes with respect to which patent portfolios the Company intends to allocate licensing and enforcement resources to in future periods. As such, certain portfolio programs were selected for termination due to a decision to no longer pursue or allocate resources, resulting in a write-off any remaining carrying value in the fourth quarter of 2015.