Allowances for estimated losses (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Balance January 1,	$ 3,942	$ 4,741	$ 2,804
(Decrease) Increase in provision	(1,680)	(799)	1,937
Balance December 31,	$ 2,262	$ 3,942	$ 4,741