Summary of significant accounting policies - Concentration of Credit Risk (Details)	12 Months Ended
	Dec. 31, 2015 USD ($) customer	Dec. 31, 2014 USD ($) customer
Concentration Risk [Line Items]
Cash, FDIC Insured Amount \| $	$ 250,000
Cash, Uninsured Amount \| $	$ 5,329,528	$ 1,272,500
Customer concentration risk \| Revenues
Concentration Risk [Line Items]
Number of customers	400
Number of customer representing more than 10% of revenues or trade accounts receivable	0	0
Customer concentration risk \| Trade accounts receivable
Concentration Risk [Line Items]
Number of customer representing more than 10% of revenues or trade accounts receivable	1
Concentration risk, percentage	16.00%	13.00%