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Intangible Assets from Acquisitions (Details Textual) (USD $)
 3 Months Ended 9 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
Dec. 31, 2011
Intangible assets from acquisitions (Textual) [Abstract]          
Original cost and accumulated amortization of fully amortized intangible assets $ 82,200,000   $ 82,200,000   $ 42,600,000
Amortization expense for intangible assets 1,100,000 1,700,000 3,700,000 3,800,000  
Estimated amortization expense, remainder of 2012 1,100,000   1,100,000    
Estimated amortization expense, 2013 2,900,000   2,900,000    
Estimated amortization expense, 2014 433,000   433,000    
Estimated amortization expense, thereafter $ 0   $ 0    
Period estimated for amortization expense     Next five years