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Business segments - Geography, Segment Reporting (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
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Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net sales | $ 433,847 | $ 400,645 | $ 388,802 | $ 365,786 | $ 339,836 | $ 333,650 | $ 326,754 | $ 318,854 | $ 1,589,080 | $ 1,319,094 | $ 1,949,857
United States [Member] |  |  |  |  |  |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net sales |  |  |  |  |  |  |  |  | 938,333 | 808,912 | 1,373,714
Long-lived assets | 370,029 |  |  |  | 395,107 |  |  |  | 370,029 | 395,107 | 408,722
Iceland [Member] |  |  |  |  |  |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net sales |  |  |  |  |  |  |  |  | 650,747 | 510,182 | 576,143
Long-lived assets | 582,981 |  |  |  | 625,897 |  |  |  | 582,981 | 625,897 | 801,268
Other [Member] |  |  |  |  |  |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Long-lived assets | $ 74,957 |  |  |  | $ 78,701 |  |  |  | $ 74,957 | $ 78,701 | $ 94,421

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- Definition Total revenue from sale of goods and services rendered during the reporting period, in the normal course of business, reduced by sales returns and allowances, and sales discounts. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.1) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 + Details
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