Summary of significant accounting policies (Tables)

12 Months Ended

Dec. 31, 2015

Accounting Policies [Abstract]

Schedule of Useful Lives

The useful lives are as follows:

	Years
Buildings	25 to 33
Plant equipment and instrumentation	2 to 10
Furniture and fixtures	4 to 8
Software	3 to 7

Components of Changes in Accumulated Other Comprehensive Income (Loss)

The components of and changes in accumulated other comprehensive income (loss) are as follows:

(U.S. Dollars in thousands)	Currency Translation Adjustments			Change in Fair Value			Accumulated Other Comprehensive Income (Loss)
Balance at December 31, 2013	$	3,651		$	(148	)	$	3,503
Unrealized gain on derivative instruments, net of tax    expense of $59		—			248			248
Currency translation adjustment (1)		(4,133	)		—			(4,133	)
Balance at December 31, 2014	$	(482	)	$	100		$	(382	)
Unrealized gain on cross-currency swaps, net of tax    expense of $74		—			128			128
Unrealized loss on debt securities (net of tax benefit     of $1,277)		—			(2,071	)		(2,071	)
Currency translation adjustment (1)		(3,907	)		—			(3,907	)
Balance at December 31, 2015	$	(4,389	)	$	(1,843	)	$	(6,232	)

(1) As the earnings generally remain indefinitely reinvested in the non U.S. dollar denominated foreign subsidiaries, no deferred taxes are recognized on the related currency translation adjustment.