Nature of business (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Nature of business
Accumulated deficit	$ (275,907)	$ (312,648)

The cumulative amount of the reporting entity's undistributed earnings or deficit.

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