Profits interest incentive plan - Allocation of Equity-based Compensation For All Class B Units (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based payment arrangement, expense	$ (6,852)	$ (6,403)
Research and Development Expense
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based payment arrangement, expense	(2,993)	(2,997)
General and Administrative Expense
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based payment arrangement, expense	(3,859)	$ (3,406)
Class B Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based payment arrangement, expense	$ 700

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5047-113901

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.F)
-URI http://asc.fasb.org/extlink&oid=122041274&loc=d3e301413-122809

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(i)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration