Summary of significant accounting policies (Details 4) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
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
Accounts Receivable | Credit concentration risk | Customer A
Concentration of credit risk
Concentration risk (as a percent)	28.00%	22.00%
Accounts Receivable | Credit concentration risk | Customer B
Concentration of credit risk
Concentration risk (as a percent)	13.00%
Accounts Receivable | Credit concentration risk | Customer C
Concentration of credit risk
Concentration risk (as a percent)	17.00%	13.00%
Accounts Receivable | Credit concentration risk | Customer D
Concentration of credit risk
Concentration risk (as a percent)		12.00%
Revenue | Customer concentration risk | Customer A
Concentration of credit risk
Concentration risk (as a percent)	21.00%	21.00%	23.00%
Revenue | Customer concentration risk | Customer C
Concentration of credit risk
Concentration risk (as a percent)		12.00%	17.00%