Subsequent events (Details) (USD $)	2 Months Ended	12 Months Ended
	Mar. 07, 2013	Dec. 31, 2012
Subsequent Event [Line Items]
Line of credit, expiration date	Mar. 07, 2015
Line of credit, fee for unused portion	0.25%	0.375%
Required minimum bank balance	$ 4,000,000	$ 4,000,000
Line of credit, minimum quick ratio	1.5