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Unrecognized tax benefits (Details 7) - USD ($) | 12 Months Ended
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Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] |  |  |
Balance at beginning of year | $ 0 | $ 105,000 | $ 7,845,000
Settlements | 0 | 0 | (7,740,000)
Lapse of statute of limitations | 0 | (105,000) | 0
Balance at end of year | 0 | 0 | 105,000
Interest (income) expense recognized in income tax expense | (92,000) | 122,000 | $ 387,000
Amount of interest income accrued for an overpayment of income taxes. | $ 54,000 |  |
Accrued liability for payment of interest |  | $ 94,000 |

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- Definition Amount of unrecognized tax benefits. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=65882285&loc=SL6600010-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Unrecognized Tax Benefit -URI http://asc.fasb.org/extlink&oid=6527854 + Details
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- Definition Amount of decrease in unrecognized tax benefits resulting from settlements with taxing authorities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=65882285&loc=SL6600010-109319 + Details
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