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Accounts Receivable
 9 Months Ended
Sep. 30, 2012
Accounts Receivable  
Accounts Receivable
2.  ACCOUNTS RECEIVABLE
   
September 30,
2012
    
December 31,
2011
  
   
(millions)
  
Utility customers
  $ 135.2     $ 124.2  
Contract services, primarily Department of Energy:
                
Billed revenue
    33.5       18.8  
Unbilled revenue
    3.2       19.0  
      36.7       37.8  
    $ 171.9     $ 162.0  
 
Accounts receivable are net of valuation allowances and allowances for doubtful accounts totaling $14.3 million at September 30, 2012 and $13.7 million at December 31, 2011.

Billings for contract services related to the U.S. Department of Energy ("DOE") are generally invoiced based on provisional billing rates approved by DOE. Unbilled revenue represents the difference between actual costs incurred, prior to incurred cost audit and notice by DOE authorizing final billing, and provisional billing rate invoiced amounts. USEC expects to invoice and collect the unbilled amounts as billing rates are revised, submitted to and approved by DOE. USEC has also invoiced certain amounts and subsequently submitted certified claims under the Contract Disputes Act for breach-of-contract amounts equaling unreimbursed costs. USEC believes DOE has breached its agreement by failing to establish appropriate provisional billing and final indirect cost rates on a timely basis.