Property, plant and equipment (Tables)

12 Months Ended

Dec. 31, 2017

Property Plant And Equipment [Abstract]

Schedule of Useful Lives of Assets

The useful lives of these assets are as follows:

	Years
Buildings	25 to 33
Plant and equipment	1 to 10
Instrumentation	3 to 4
Computer software	3 to 7
Furniture and fixtures	4 to 8

Schedule of Property Plant and Equipment

The assets and related accumulated depreciation are adjusted for property retirements and disposals, with the resulting gain or loss included in earnings. Fully depreciated assets remain in the accounts until retired from service.

	December 31,
(U.S. Dollars, in thousands)	2017			2016
Cost
Buildings	$	3,725		$	3,225
Plant and equipment		47,588			43,745
Instrumentation		75,818			71,962
Computer software		48,604			44,720
Furniture and fixtures		7,605			8,308
Construction in progress		769			1,907
		184,109			173,867
Accumulated depreciation		(138,970	)		(124,951	)
Property, plant and equipment, net	$	45,139		$	48,916