Stock based compensation (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended					0 Months Ended	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 Stock Appreciation Rights (SARs) [Member]	Dec. 31, 2011 Restricted Stock Units (RSUs) [Member]	May 26, 2011 Deferred Stock Units [Member]	Dec. 31, 2011 Deferred Stock Units [Member]	Dec. 31, 2011 Performance Shares [Member]
Stock-Based Compensation [Abstract]
Shares available for future grant under the plan	893,952
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number	0
Share Based Compensation Arrangement By Share Based Payment Award Non Option Equity Instruments Granted Per Director						131
Expense recognized in connection with the issuance of Deferred Stock Units	$ 300	$ 0	$ 0				$ 300
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Outstanding Number				0	0		1,048	0