Income taxes (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Income tax computed at federal statutory tax rate	34.00%	34.00%	34.00%
State taxes, net of federal benefit	6.30%	4.00%	5.50%
General business credit carryovers	2.40%	3.20%	2.20%
Non‑deductible expenses	(0.70%)	(15.60%)	(5.00%)
Change in valuation allowance	(42.00%)	(25.60%)	(36.70%)
Deferred tax assets:
Net operating loss carryforwards		$ 12,521	$ 6,643
Tax credit carryforwards		1,969	673
Deferred rent		516	626
Non‑deductible expenses		577	79
Intangibles		376	327
Stock Compensation		115
Total deferred tax assets		16,074	8,348
Less valuation allowance		(15,207)	(7,599)
Net deferred tax assets		867	749
Deferred tax liabilities—depreciation and amortization		(867)	(749)
Net deferred taxes		$ 0	$ 0