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Fair value measurements (Tables) | 12 Months Ended
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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 |

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- Definition Tabular disclosure of the reconciliation of available-for-sale securities from cost basis to fair value. + References No definition available. + Details
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- Definition Tabular disclosure of assets and liabilities, including [financial] instruments measured at fair value that are classified in stockholders' equity, if any, that are measured at fair value on a recurring basis. The disclosures contemplated herein include the fair value measurements at the reporting date by the level within the fair value hierarchy in which the fair value measurements in their entirety fall, segregating fair value measurements using quoted prices in active markets for identical assets (Level 1), significant other observable inputs (Level 2), and significant unobservable inputs (Level 3). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19190-110258 + Details
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