Income taxes - Operating loss carryforwards - (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 12,521	$ 6,643
Valuation Allowance	15,207	7,599
Change in the valuation allowance	7,600
Unrecognized tax benefits	0	$ 0
Federal | Tax Year 2028
Operating Loss Carryforwards [Line Items]
Research and development tax credit carryforwards	1,400
Federal | Tax Year 2033
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	32,100
State | Tax Year 2028
Operating Loss Carryforwards [Line Items]
Research and development tax credit carryforwards	900
State | Tax Year 2033
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 30,200