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Summary of significant accounting policies (Allowance for doubtful accounts) (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Allowance for Doubtful Accounts Receivable [Roll Forward]      
Balance at beginning of period $ 5,891 $ 5,795 $ 4,125
Charged to expenses 2,925 2,115 2,867
Deductions or other (3,091) (2,019) (1,197)
Balance at end of period $ 5,725 $ 5,891 $ 5,795