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

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

Name:	us-gaap_MovementInValuationAllowancesAndReservesRollForward
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- 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

Name:	us-gaap_ValuationAllowancesAndReservesBalance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of increase in valuation and qualifying accounts and reserves from charge to cost and expense.

+ References

+ Details

Name:	us-gaap_ValuationAllowancesAndReservesChargedToCostAndExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of decrease in valuation and qualifying accounts and reserves.

+ References

+ Details

Name:	us-gaap_ValuationAllowancesAndReservesDeductions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_ValuationAllowancesAndReservesTypeAxis=us-gaap_SalesReturnsAndAllowancesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: