Fair value measurements (Tables)

12 Months Ended

Dec. 31, 2017

Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]

Schedule of Financial Assets and Liabilities Recorded at Fair Value on Recurring Basis

(U.S. Dollars, in thousands)	Balance December 31, 2017			Level 1		Level 2			Level 3
Assets
Collective trust funds	$	100		$	—	$	100		$	—
Treasury securities		556			556		—			—
Debt security		16,050			—		—			16,050
Total	$	16,706		$	556	$	100		$	16,050
Liabilities
Deferred compensation plan	$	(1,379	)	$	—	$	(1,379	)	$	—
Total	$	(1,379	)	$	—	$	(1,379	)	$	—

(U.S. Dollars, in thousands)	Balance December 31, 2016			Level 1		Level 2			Level 3
Assets
Collective trust funds	$	1,584		$	—	$	1,584		$	—
Treasury securities		467			467		—			—
Certificates of deposit		468			468		—			—
Debt security		12,220			—		—			12,220
Total	$	14,739		$	935	$	1,584		$	12,220
Liabilities
Deferred compensation plan	$	(1,452	)	$	—	$	(1,452	)	$	—
Total	$	(1,452	)	$	—	$	(1,452	)	$	—

Fair Value, Inputs, Level 3 [Member]

Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]

Schedule of Reconciliation of Available for Sale Debt Securities

The following table provides a reconciliation of the beginning and ending balances for debt securities measured at fair value using significant unobservable inputs (Level 3):

(U.S. Dollars, in thousands)	2017			2016
Balance at January 1	$	12,220		$	12,658
Accrued interest income		—			1,306
Gains or losses recorded for the period
Recognized in net income		(5,585	)		(2,727	)
Recognized in other comprehensive income		9,415			983
Balance at December 31	$	16,050		$	12,220