Patents and other intangible assets

12 Months Ended

Dec. 31, 2017

Goodwill And Intangible Assets Disclosure [Abstract]

4.	Patents and other intangible assets

Patents and other intangible assets are recorded at cost, or when acquired as a part of a business combination at estimated fair value. These assets are amortized on a straight-line basis over the useful lives of the assets. The Company’s weighted average amortization period for developed technologies is 11 years.

	December 31,
(U.S. Dollars, in thousands)	2017			2016
Cost
Patents	$	38,621		$	38,348
License and other		10,276			7,611
Trademarks—finite lived		533			319
		49,430			46,278
Accumulated amortization
Patents		(34,151	)		(34,717	)
License and other		(4,625	)		(3,962	)
Trademarks—finite lived		(193	)		(138	)
		(38,969	)		(38,817	)
Patents and other intangible assets, net	$	10,461		$	7,461

Amortization expense for intangible assets was $1.8 million, $1.8 million and $1.7 million for the years ended December 31, 2017, 2016 and 2015, respectively. Future amortization expense for intangible assets is estimated as follows:

(U.S. Dollars, in thousands)	Amortization
2018	$	2,352
2019		1,909
2020		1,366
2021		1,321
2022		1,314
Thereafter		2,199
Total	$	10,461