Income taxes - Tax Carry-forwards and Unrecognized Tax Benefits (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Unrecognized tax benefits
Accrued interest related to uncertain tax positions	$ 0
Roll forward of gross unrecognized tax benefits
Beginning Balance	3,956,000	$ 7,261,000
Increase based on tax positions taken during a current period	6,872,000	5,458,000
Decrease based on tax positions taken during the current period	(2,182,000)	(8,763,000)
Ending Balance	8,646,000	3,956,000
Federal
Tax carryforwards and uncertainties
Net operating loss carry-forwards	280,072,000
Net operating loss carry-forwards with no expiration date	$ 34,611,000
Percentage of taxable income which limits the utilization of net operating loss carryforwards	80.00%
Net operating loss carry-forward subject to expiration beginning 2024	$ 245,461,000
Net operating loss carryforward that will expire unused	98,009,000
State
Tax carryforwards and uncertainties
Net operating loss carry-forwards	2,958,000	$ 3,166,000
Research and development credits
Tax carryforwards and uncertainties
Tax credits carry-forwards	2,686,000
Research and development credits | Federal
Tax carryforwards and uncertainties
Tax credit carryforwards that will expire unused	$ 350,000