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Summary of significant accounting policies - Reconciliation of Sales Allowances (Details) - Sales Allowances - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Sales Allowances [Roll Forward] |  |  |
Beginning balance | $ 8,458 | $ 11,927 | $ 3,866
Charges | 26,971 | 25,680 | 24,427
Deductions | (27,655) | (29,149) | (16,366)
Ending balance | $ 7,774 | $ 8,458 | $ 11,927

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- Definition A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period. + References No definition available. + Details
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- Definition Amount of valuation and qualifying accounts and reserves. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 4 -Subparagraph (SX 210.12-09) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24092-122690 + Details
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