Investments in unconsolidated companies - Summarized income statement for investments in unconsolidated companies (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Equity Method Investment, Summarized Financial Information, Income Statement
Revenue	$ 55,135	$ 47,794	$ 94,600	$ 86,352
Gross profit	39,452	32,921	64,971	58,765
Net earnings	$ 6,721	$ 4,287	$ 7,548	$ 6,091

The amount of net income (loss) reported by an equity method investment of the entity.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721683-107760

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 28
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
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-URI http://asc.fasb.org/extlink&oid=66011266&loc=d3e33918-111571

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section S99
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Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
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-Section 50
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-URI http://asc.fasb.org/extlink&oid=66011266&loc=d3e33912-111571

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