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Income taxes Components of unrecognized tax positions (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]        
Highly certain tax positions $ 1,100 $ 16,900 $ 15,100  
Other unrecognized tax benefits 100 700 800  
Gross unrecognized tax benefits 1,200 17,600 15,900 16,600
Accrued interest and penalties related to unrecognized tax benefits $ 100 $ 100 $ 100