Commitments and contingencies (Narrative) (Details) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Commitments and contingencies [Abstract]
Rent expenses	$ 309,502	$ 276,433	$ 928,351	$ 818,652
Commitments and Contingencies Disclosure [Line Items]
Contracted capital commitments			$ 2,032,800		$ 1,755,387
Minimum [Member]
Commitments and Contingencies Disclosure [Line Items]
Lease expiration year			2012
Maximum [Member]
Commitments and Contingencies Disclosure [Line Items]
Lease expiration year			2016