Income taxes (Details 2)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income taxes
State tax, High End	0.20%	0.20%	0.20%
Reconciliation from the U.S. Federal statutory income rate of 34.0% to the effective income tax rate
Federal statutory rate	34.00%	34.00%	34.00%
Permanent differences	(4.80%)	15.10%	(12.20%)
Foreign tax rates at rates different from U.S. rates	18.80%	(48.90%)	21.10%
Valuation allowance	(51.40%)	12.00%	(43.00%)
Total tax provision	(3.40%)	12.20%	(0.10%)