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Segment reporting - Summary of information of long-lived assets in different geographic areas (Details) - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | --- | ---
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |
Net property, plant and equipment | $ 514,741 | $ 512,841 | $ 508,855
U.S. and Canada |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |
Net property, plant and equipment |  | 177,298 | 191,502
Mexico |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |
Net property, plant and equipment |  | 147,959 | 151,288
Brazil |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |
Net property, plant and equipment |  | 3,547 | 6,100
France |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |
Net property, plant and equipment |  | 80,035 | 69,558
Spain |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |
Net property, plant and equipment |  | 103,819 | 87,614
South Africa |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |
Net property, plant and equipment |  | 20 | 2,547
Other countries |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |
Net property, plant and equipment |  | $ 163 | $ 246

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- Definition Amount after accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(13)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(14)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(8)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229 + Details
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