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Property and equipment (Details textual) (USD $)
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Depreciation, Depletion and Amortization $ 3,897,000 $ 3,414,000 $ 3,703,000
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment 31,487,000 28,868,000  
Property Plant and Equipment [Member]
      
Depreciation, Depletion and Amortization 3,100,000 2,800,000 3,100,000
Equipment [Member]
      
Capital Leased Assets, Gross 1,600,000 1,600,000  
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment $ 1,600,000 $ 1,500,000