Fair value measurements (Tables)

12 Months Ended

Dec. 31, 2015

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, 2015			Level 1		Level 2			Level 3
Assets
Collective trust funds	$	1,622		$	—	$	1,622		$	—
Treasury securities		495			495		—			—
Certificates of deposit		337			337		—			—
Derivative securities		2,485			—		2,485			—
Debt securities		12,658			—		—			12,658
Total	$	17,597		$	832	$	4,107		$	12,658
Liabilities
Deferred compensation plan	$	(1,503	)	$	—	$	(1,503	)	$	—
Total	$	(1,503	)	$	—	$	(1,503	)	$	—

(U.S. Dollars in thousands)	Balance December 31, 2014			Level 1		Level 2			Level 3
Assets
Collective trust funds	$	1,696		$	—	$	1,696		$	—
Treasury securities		586			586		—			—
Certificates of deposit		1,510			1,510		—			—
Derivative securities		2,504			—		2,504			—
Total	$	6,296		$	2,096	$	4,200		$	—
Liabilities
Deferred compensation plan	$	(1,886	)	$	—	$	(1,886	)	$	—
Total	$	(1,886	)	$	—	$	(1,886	)	$	—

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)	2015
Balance at January 1, 2015	$	—
Additions to debt securities		15,000
Accrued interest income		1,006
Unrealized loss on debt securities		(3,348	)
Balance at December 31, 2015	$	12,658