Mergers, Acquisitions and Dispositions Merger Commitments Table (Details) - USD ($) $ in Millions	Dec. 31, 2017	Mar. 31, 2017
Other Commitments [Line Items]
Other Commitment	$ 137
Pepco Holdings LLC [Member]
Other Commitments [Line Items]
Other Commitment		$ 444
Potomac Electric Power Company [Member]
Other Commitments [Line Items]
Other Commitment	86
Delmarva Power and Light Company [Member]
Other Commitments [Line Items]
Other Commitment	27
Atlantic City Electric Company [Member]
Other Commitments [Line Items]
Other Commitment	121
Pepco Holdings LLC [Member]
Other Commitments [Line Items]
Other Commitment	513
Pepco Holdings LLC [Member] | Potomac Electric Power Company [Member]
Other Commitments [Line Items]
Other Commitment	120
Pepco Holdings LLC [Member] | Delmarva Power and Light Company [Member]
Other Commitments [Line Items]
Other Commitment	84
Pepco Holdings LLC [Member] | Atlantic City Electric Company [Member]
Other Commitments [Line Items]
Other Commitment	111
Pepco Holdings LLC [Member] | Rate Bill Credits [Member]
Other Commitments [Line Items]
Other Commitment	259
Pepco Holdings LLC [Member] | Rate Bill Credits [Member] | Potomac Electric Power Company [Member]
Other Commitments [Line Items]
Other Commitment	91
Pepco Holdings LLC [Member] | Rate Bill Credits [Member] | Delmarva Power and Light Company [Member]
Other Commitments [Line Items]
Other Commitment	67
Pepco Holdings LLC [Member] | Rate Bill Credits [Member] | Atlantic City Electric Company [Member]
Other Commitments [Line Items]
Other Commitment	101
Pepco Holdings LLC [Member] | Energy Efficiency Program [Member]
Other Commitments [Line Items]
Other Commitment	122
Pepco Holdings LLC [Member] | Energy Efficiency Program [Member] | Potomac Electric Power Company [Member]
Other Commitments [Line Items]
Other Commitment	0
Pepco Holdings LLC [Member] | Energy Efficiency Program [Member] | Delmarva Power and Light Company [Member]
Other Commitments [Line Items]
Other Commitment	0
Pepco Holdings LLC [Member] | Energy Efficiency Program [Member] | Atlantic City Electric Company [Member]
Other Commitments [Line Items]
Other Commitment	0
Pepco Holdings LLC [Member] | Charitable Contributions [Member]
Other Commitments [Line Items]
Other Commitment	50
Pepco Holdings LLC [Member] | Charitable Contributions [Member] | Potomac Electric Power Company [Member]
Other Commitments [Line Items]
Other Commitment	28
Pepco Holdings LLC [Member] | Charitable Contributions [Member] | Delmarva Power and Light Company [Member]
Other Commitments [Line Items]
Other Commitment	12
Pepco Holdings LLC [Member] | Charitable Contributions [Member] | Atlantic City Electric Company [Member]
Other Commitments [Line Items]
Other Commitment	10
Pepco Holdings LLC [Member] | Delivery System Modernization [Member]
Other Commitments [Line Items]
Other Commitment	22
Pepco Holdings LLC [Member] | Delivery System Modernization [Member] | Potomac Electric Power Company [Member]
Other Commitments [Line Items]
Other Commitment	0
Pepco Holdings LLC [Member] | Delivery System Modernization [Member] | Delmarva Power and Light Company [Member]
Other Commitments [Line Items]
Other Commitment	0
Pepco Holdings LLC [Member] | Delivery System Modernization [Member] | Atlantic City Electric Company [Member]
Other Commitments [Line Items]
Other Commitment	0
Pepco Holdings LLC [Member] | Green Sustainability Fund [Member]
Other Commitments [Line Items]
Other Commitment	14
Pepco Holdings LLC [Member] | Green Sustainability Fund [Member] | Potomac Electric Power Company [Member]
Other Commitments [Line Items]
Other Commitment	0
Pepco Holdings LLC [Member] | Green Sustainability Fund [Member] | Delmarva Power and Light Company [Member]
Other Commitments [Line Items]
Other Commitment	0
Pepco Holdings LLC [Member] | Green Sustainability Fund [Member] | Atlantic City Electric Company [Member]
Other Commitments [Line Items]
Other Commitment	0
Pepco Holdings LLC [Member] | Workforce Development [Member]
Other Commitments [Line Items]
Other Commitment	17
Pepco Holdings LLC [Member] | Workforce Development [Member] | Potomac Electric Power Company [Member]
Other Commitments [Line Items]
Other Commitment	0
Pepco Holdings LLC [Member] | Workforce Development [Member] | Delmarva Power and Light Company [Member]
Other Commitments [Line Items]
Other Commitment	0
Pepco Holdings LLC [Member] | Workforce Development [Member] | Atlantic City Electric Company [Member]
Other Commitments [Line Items]
Other Commitment	0
Pepco Holdings LLC [Member] | Other1 [Member]
Other Commitments [Line Items]
Other Commitment	29
Pepco Holdings LLC [Member] | Other1 [Member] | Potomac Electric Power Company [Member]
Other Commitments [Line Items]
Other Commitment	1
Pepco Holdings LLC [Member] | Other1 [Member] | Delmarva Power and Light Company [Member]
Other Commitments [Line Items]
Other Commitment	5
Pepco Holdings LLC [Member] | Other1 [Member] | Atlantic City Electric Company [Member]
Other Commitments [Line Items]
Other Commitment	0
Successor [Member] | Pepco Holdings LLC [Member] | Pepco Holdings LLC [Member]
Other Commitments [Line Items]
Other Commitment	315
Successor [Member] | Pepco Holdings LLC [Member] | Rate Bill Credits [Member] | Pepco Holdings LLC [Member]
Other Commitments [Line Items]
Other Commitment	259
Successor [Member] | Pepco Holdings LLC [Member] | Energy Efficiency Program [Member] | Pepco Holdings LLC [Member]
Other Commitments [Line Items]
Other Commitment	0
Successor [Member] | Pepco Holdings LLC [Member] | Charitable Contributions [Member] | Pepco Holdings LLC [Member]
Other Commitments [Line Items]
Other Commitment	50
Successor [Member] | Pepco Holdings LLC [Member] | Delivery System Modernization [Member] | Pepco Holdings LLC [Member]
Other Commitments [Line Items]
Other Commitment	0
Successor [Member] | Pepco Holdings LLC [Member] | Green Sustainability Fund [Member] | Pepco Holdings LLC [Member]
Other Commitments [Line Items]
Other Commitment	0
Successor [Member] | Pepco Holdings LLC [Member] | Workforce Development [Member] | Pepco Holdings LLC [Member]
Other Commitments [Line Items]
Other Commitment	0
Successor [Member] | Pepco Holdings LLC [Member] | Other1 [Member] | Pepco Holdings LLC [Member]
Other Commitments [Line Items]
Other Commitment	$ 6