Stock based compensation (Details) (USD $)	12 Months Ended			1 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	May 31, 2012 Deferred Stock Units [Member]	Dec. 31, 2012 Deferred Stock Units [Member]
Stock-Based Compensation [Abstract]
Shares available for future grant under the plan	892,920
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share Based Compensation Arrangement By Share Based Payment Award Non Option Equity Instruments Granted Per Director (in shares)				129
Expense recognized in connection with the issuance of Deferred Stock Units	$ 300,000	$ 300,000	$ 0	$ 300,000
Expense per grant recognized in connection with the issuance of Deferred Stock Units				$ 37,500
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Outstanding Number					2,080