Income taxes - Net operating loss carryforwards (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2016 item
Operating Loss Carryforwards [Line Items]
Maximum NOL carryforwards (as a percent)	80.00%
Valuation Allowance	$ 84,866	$ 90,920	$ 90,900
Change in the valuation allowance	$ (1,000)
Number of ownership changes | item				3
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Income tax computed at federal statutory tax rate	21.00%	21.00%	21.00%
Nondeductible expenses	5.00%	(2.20%)	(2.10%)
Other	1.90%
State taxes, net of federal benefit	(2.30%)	8.00%	6.30%
Change in valuation allowance	(5.00%)	(36.20%)	(28.10%)
General business credit carryovers	(20.60%)	9.40%	3.10%
Total			0.20%
Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 153,400
State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	140,900
Investment tax credit	900
Research and development tax credits carryforwards | Federal
Operating Loss Carryforwards [Line Items]
Tax credits carryforwards	21,100
Research and development tax credits carryforwards | State
Operating Loss Carryforwards [Line Items]
Tax credits carryforwards	$ 8,300