Commitments and contingencies (Narrative) (Details) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Commitments and contingencies [Abstract]
Rent expenses	$ 301,483	$ 308,603	$ 630,382	$ 618,849
Commitments and Contingencies Disclosure [Line Items]
Contracted capital commitments					$ 791,934
Minimum [Member]
Commitments and Contingencies Disclosure [Line Items]
Lease expiration year			2013
Maximum [Member]
Commitments and Contingencies Disclosure [Line Items]
Lease expiration year			2016