Commitments and contingencies - Narrative (Details) - USD ($)		1 Months Ended	10 Months Ended	12 Months Ended
	Jan. 27, 2017	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Long-term Purchase Commitment [Line Items]
Rent expense			$ 152,000	$ 337,000	$ 253,000
Percent of gross sales due to related parties		2.00%				2.00%
Royalty expense				20,000	0
Royalties payable			$ 0	$ 4,000	$ 0	$ 0
Subsequent Event | 800 Boylston St
Long-term Purchase Commitment [Line Items]
Operating leases, increase in future minimum payments due	$ 933,186