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Accounts Receivable (Tables)
9 Months Ended
Sep. 30, 2012
Accounts Receivable  
Schedule of 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