Concentration of credit risk (Details) (USD $)			6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended		6 Months Ended	3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2013 Accounts Receivable Credit concentration risk Customer A	Dec. 31, 2012 Accounts Receivable Credit concentration risk Customer A	Jun. 30, 2013 Accounts Receivable Credit concentration risk Customer B	Dec. 31, 2012 Accounts Receivable Credit concentration risk Customer B	Dec. 31, 2012 Accounts Receivable Credit concentration risk Customer C	Jun. 30, 2013 Accounts Receivable Credit concentration risk Customer D	Jun. 30, 2013 Revenue Customer concentration risk Customer A	Jun. 30, 2012 Revenue Customer concentration risk Customer A	Jun. 30, 2013 Revenue Customer concentration risk Customer A	Jun. 30, 2012 Revenue Customer concentration risk Customer A	Jun. 30, 2013 Revenue Customer concentration risk Customer C	Jun. 30, 2012 Revenue Customer concentration risk Customer C	Jun. 30, 2013 Revenue Customer concentration risk Customer C	Jun. 30, 2012 Revenue Customer concentration risk Customer C
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
Cash balances at foreign banking institutions which are not insured by the FDIC	$ 4,700,000	$ 5,300,000
Concentration risk (as a percent)			19.00%	26.00%	12.00%	14.00%	11.00%	22.00%	20.00%	24.00%	20.00%	23.00%	13.00%	16.00%	15.00%	18.00%