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Concentration of Credit Risk and Off-Balance Sheet Risk (Tables)
 9 Months Ended
Sep. 30, 2013
Risks And Uncertainties [Abstract]  
Revenue and Trade Accounts Receivable from Bidders

The following represents revenue and trade accounts receivable from bidders exceeding 10% of the total in each category:

 

     Revenue for the three
months ended September 30,		     Revenue for the nine
months ended September 30,		     Trade Accounts Receivable as
of September 30,		  
Bidder    2013     2012     2013     2012     2013     2012  

A

      6 %      8 %      8 %      10 %      8 %      8 % 

B

      13 %      12 %      12 %      12 %      14 %      13 % 

C

      8 %      3 %      8 %      3 %      10 %      5 %