Acacia’s only identifiable intangible assets are patents and patent rights, with estimated remaining economic useful lives ranging from one to eight 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, 2015 and 2014 are as follows (in thousands):



	2015			2014

Gross carrying amount - patents	$	444,137		$	453,201
Accumulated amortization - patents⁽¹⁾	(281,495		)	(166,565		)
Patents, net	$	162,642		$	286,636

(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 5 years. Scheduled annual aggregate amortization expense is estimated to be $43,218,000 in 2016, $40,577,000 in 2017, $35,777,000 in 2018, $19,410,000 in 2019, $6,787,000 in 2020 and $16,873,000 thereafter.

For the years ended December 31, 2015, 2014 and 2013, Acacia paid patent investment costs totaling $19,504,000, $42,746,000 and $25,061,000, respectively. The patents have initial estimated economic useful lives ranging from two to ten years. Included in net additions to capitalized patent costs during the years ended December 31, 2015 and 2014 are accrued patent investment costs totaling $1,000,000 and $16,700,000, respectively, which are amortized over the estimated economic useful life of the related patents.

During the periods presented, certain operating subsidiaries recovered up-front patent portfolio advances from applicable net licensing proceeds prior to the scheduled amortization of such up-front patent portfolio advances, resulting in the acceleration of amortization expense for the applicable patent-related assets. For the years ended December 31, 2014 and 2013, accelerated amortization expense related to the recovery of up-front patent portfolio advances totaled $1,247,000 and $592,000, respectively.

For the years ended December 31, 2015, 2014 and 2013, pursuant to the terms of the respective inventor agreements, certain Acacia operating subsidiaries elected to terminate or sell their rights to patent portfolios, resulting in the acceleration of amortization expense for the patent-related assets totaling $380,000, $2,702,000 and $1,747,000, respectively.

Acacia recorded impairment of patent-related intangible asset charges totaling $74,731,000, $3,497,000 and $4,619,000 for the years ended December 31, 2015, 2014 and 2013, 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 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 consisted of the excess of the asset’s carrying value over its estimated fair value. Refer to Note 7 for additional information regarding impairment charges for the year ended December 31, 2015.

For the years ended December 31, 2015, 2014 and 2013, capitalized patent costs, accumulated amortization, and sales proceeds related to patent-related sales and disposals are as follows (in thousands):



	2015		2014		2013

Capitalized patent costs	$	500	$	3,000	$	3,500
Accumulated amortization	120		298		1,753
Sales proceeds	750		3,500		1,000