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Segment Information - Narrative (Details)
9 Months Ended
Sep. 30, 2017
Reportable_segment
Segment Reporting Information [Line Items]  
Number of reportable segments 12
Exelon Generation Co L L C [Member]  
Segment Reporting Information [Line Items]  
Number of reportable segments 6
Pepco Holdings LLC [Member]  
Segment Reporting Information [Line Items]  
Number of reportable segments 3