Concentration of credit risk (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Concentration of credit risk
Cash, cash equivalents, and restricted cash with financial banking institutions that are insured by the Federal Deposit Insurance Corporation ("FDIC")	$ 250,000		$ 250,000
Accounts Receivable | Credit concentration risk | Customer A
Concentration of credit risk
Concentration risk (as a percent)			18.00%		22.00%
Accounts Receivable | Credit concentration risk | Customer B
Concentration of credit risk
Concentration risk (as a percent)			12.00%		17.00%
Revenue | Customer concentration risk | Customer A
Concentration of credit risk
Concentration risk (as a percent)	18.00%	16.00%	19.00%	16.00%
Revenue | Customer concentration risk | Customer G
Concentration of credit risk
Concentration risk (as a percent)		14.00%		12.00%