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Segment Information (Narrative) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Segment Reporting [Abstract]    
Revenue from top 10 customers 97.00% 90.00%
Revenue from top 4 customers 53.00% 50.00%
Revenue from first customer over 10 percent 16.00% 15.00%
Revenue from second customer over 10 percent 14.00% 12.00%
Revenue from third customer over 10 percent 12.00% 12.00%
Revenue from fourth customer over 10 percent 11.00% 11.00%
Revenue from fifth customer over 10 percent 11.00%  
Assets LEU segment $ 657.4 $ 686.0
Assets contract services segment 17.9 27.5
Assets $ 675.3 $ 713.5