Summary of significant accounting policies - Concentration of credit risk (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Concentration Risk [Line Items]
Cash, cash equivalents, and restricted cash with financial banking institutions that are insured by the Federal Deposit Insurance Corporation ("FDIC")	$ 250,000
Accounts Receivable | Credit concentration risk | Customer A
Concentration Risk [Line Items]
Concentration Risk, Percentage	22.00%	15.00%
Accounts Receivable | Credit concentration risk | Customer B
Concentration Risk [Line Items]
Concentration Risk, Percentage	17.00%
Accounts Receivable | Credit concentration risk | Customer H
Concentration Risk [Line Items]
Concentration Risk, Percentage		11.00%
Revenue | Customer concentration risk | Customer A
Concentration Risk [Line Items]
Concentration Risk, Percentage	17.00%	18.00%	21.00%
Revenue | Customer concentration risk | Customer H
Concentration Risk [Line Items]
Concentration Risk, Percentage		11.00%
Revenue | Customer concentration risk | Customer G
Concentration Risk [Line Items]
Concentration Risk, Percentage	12.00%