Concentration of credit risk (Details) (USD $)			3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended	3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Accounts Receivable Credit concentration risk Customer A	Dec. 31, 2012 Accounts Receivable Credit concentration risk Customer A	Mar. 31, 2013 Accounts Receivable Credit concentration risk Customer B	Dec. 31, 2012 Accounts Receivable Credit concentration risk Customer B	Mar. 31, 2013 Accounts Receivable Credit concentration risk Customer C	Dec. 31, 2012 Accounts Receivable Credit concentration risk Customer C	Mar. 31, 2013 Accounts Receivable Credit concentration risk Customer D	Mar. 31, 2013 Revenue Customer concentration risk Customer A	Mar. 31, 2012 Revenue Customer concentration risk Customer A	Mar. 31, 2013 Revenue Customer concentration risk Customer C	Mar. 31, 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	$ 4,500,000	$ 5,300,000
Concentration risk (as a percent)			23.00%	26.00%	14.00%	14.00%	12.00%	11.00%	14.00%	20.00%	23.00%	17.00%	20.00%