Income taxes - Net operating loss carryforwards (Details) $ in Thousands		12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 item
Operating Loss Carryforwards [Line Items]
Valuation Allowance	$ 44,953	$ 75,213	$ 44,953
Change in the valuation allowance		$ 30,200
Number of ownership changes | item				3
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Income tax computed at federal statutory tax rate	35.00%	21.00%	34.00%	34.00%
State taxes, net of federal benefit		6.30%	6.10%	5.60%
General business credit carryovers		3.10%	5.00%	4.20%
Non‑deductible expenses		(2.10%)	(4.10%)	(4.00%)
Deferred rate change			(21.80%)
Change in valuation allowance		(28.10%)	(19.20%)	(40.20%)
Total		0.20%		(0.40%)
Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards		$ 162,900
Maximum NOL carryforwards (as a percent)		80.00%
State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards		$ 163,800
Investment Tax Credit expiring in 2018		400
Tax Year 2018 | Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards		73,100
Tax Year 2018 | State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards		72,100
Research and development tax credits carryforwards | Federal
Operating Loss Carryforwards [Line Items]
Tax credits carryforwards		9,400
Research and development tax credits carryforwards | State
Operating Loss Carryforwards [Line Items]
Tax credits carryforwards		$ 3,000