Income taxes - Tax Carry-forwards and Unrecognized Tax Benefits (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Unrecognized tax benefits
Accrued interest related to uncertain tax positions	$ 0
Roll forward of gross unrecognized tax benefits
Beginning Balance	7,261,000	$ 5,081,000
Increase/(Decrease) based on tax positions taken during a prior period	(3,305,000)	2,180,000
Ending Balance	3,956,000	7,261,000
Federal
Tax carryforwards and uncertainties
Net operating loss carry-forwards	275,628,000
Net operating loss carryforward that will expire unused		98,009,000
State
Tax carryforwards and uncertainties
Net operating loss carry-forwards	3,166,000	$ 3,136,000
Research and development credits
Tax carryforwards and uncertainties
Tax credits carry-forwards	2,493,000
Research and development credits | Federal
Tax carryforwards and uncertainties
Tax credit carryforwards that will expire unused	$ 350,000