Mergers, Acquisitions and Dispositions Merger Integration Related Costs Table (Details) - USD ($) $ in Millions		3 Months Ended	9 Months Ended	12 Months Ended
		Mar. 23, 2016	Dec. 31, 2016	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016	Mar. 31, 2017
Other Commitment				$ 127
Business Combination, Integration Related Costs				7	[1]	$ 16	[1]	$ 143
Exelon Generation Co L L C [Member]
Business Combination, Integration Related Costs				5	[1]	22	[1]	37
PECO Energy Co [Member]
Business Combination, Integration Related Costs				1	[1]	4	[1]	5
Baltimore Gas and Electric Company [Member]
Business Combination, Integration Related Costs				1	[1],[2]	4	[1],[2]	(1)
Pepco Holdings LLC [Member]
Other Commitment									$ 444
Business Combination, Integration Related Costs				0	[1],[2]	(6)	[1],[2]	28
Delmarva Power & Light Company [Member]
Business Combination, Integration Related Costs				0	[1],[2]	(7)	[1],[2]	20
Atlantic City Electric Company [Member]
Business Combination, Integration Related Costs				0	[1],[2]	(6)	[1],[2]	19
Commonwealth Edison Co [Member]
Business Combination, Integration Related Costs				0	[1],[2]	1	[1],[2]	$ (6)
Predecessor [Member] | Pepco Holdings LLC [Member]
Business Combination, Integration Related Costs	[2]	$ 29
Successor [Member] | Pepco Holdings LLC [Member]
Business Combination, Integration Related Costs	[2]		$ 69	0		(18)	[1]
Regulatory Assets [Member] | Baltimore Gas and Electric Company [Member]
Business Combination, Integration Related Costs			6
Regulatory Assets [Member] | Pepco Holdings LLC [Member]
Business Combination, Integration Related Costs			16			24
Regulatory Assets [Member] | Potomac Electric Power Company [Member]
Business Combination, Integration Related Costs			11			8
Regulatory Assets [Member] | Delmarva Power & Light Company [Member]
Business Combination, Integration Related Costs			4			8
Regulatory Assets [Member] | Atlantic City Electric Company [Member]
Business Combination, Integration Related Costs						$ 8
Regulatory Assets [Member] | Commonwealth Edison Co [Member]
Business Combination, Integration Related Costs			$ 8
Potomac Electric Power Company [Member] | Remaining Commitment [Member]
Other Commitment				$ 73

[1] The costs incurred are classified primarily within Operating and maintenance expense in the Registrants’ respective Consolidated Statements of Operations and Comprehensive Income, with the exception of the financing costs, which are included within Interest expense. Costs do not include merger commitments discussed above.

[2] (b)For the year ended December 31, 2017, includes deferrals of previously incurred integration costs as regulatory assets of $24 million, $8 million, $8 million, and $8 million at PHI, Pepco, DPL and ACE, respectively. For the year ended December 31, 2016, includes deferrals of previously incurred integration costs as regulatory assets of $8 million, $6 million, $11 million and $4 million at ComEd, BGE, Pepco and DPL, respectively. For the Successor period March 24, 2016 to December 31, 2016, includes deferrals of previously incurred integration costs as regulatory assets of $16 million at PHI. See Note 4 - Regulatory Matters for additional information.