Significant agreements - Revenue recognition due to changes in contract assets and liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Significant Agreements
Revenue recognized based on proportional performance	$ (429)	$ (4,472)	$ (355)
Revenue recognized based on expiration of material rights	(9,984)
Total	$ (10,413)	$ (4,472)	$ (355)

Amount of revenue recognized that was previously included in balance of obligation to transfer good or service to customer for which consideration from customer has been received or is due.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130549-203045

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