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v3.20.1

Summary of significant accounting policies - Reconciliation of Sales Allowances (Details) - Sales Allowances - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2019 | Mar. 31, 2020 | Dec. 31, 2018 | Dec. 31, 2017
Sales Allowances [Roll Forward] |  |  |  |
Beginning balance | $ 7,774 | $ 6,545 | $ 8,458 | $ 11,927
Charges | 6,787 | 29,576 | 26,971 | 25,680
Deductions | (8,016) | (28,508) | (27,655) | (29,149)
Ending balance | $ 6,545 | $ 7,613 | $ 7,774 | $ 8,458

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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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