Significant Accounting Policies - Summary of Capitalized Software Costs (Details) - USD ($) $ in Millions		3 Months Ended	9 Months Ended	12 Months Ended
		Mar. 23, 2016	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Capitalized Software [Line Items]
Net unamortized software costs	[1]		$ 808	$ 834	$ 808
Amortization of capitalized software costs	[1]			270	255	$ 208
Pepco Holdings LLC [Member]
Capitalized Software [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Net Income					34
Net unamortized software costs			153	133	153
Exelon Generation Co L L C [Member]
Capitalized Software [Line Items]
Net unamortized software costs	[1]		173	173	173
Amortization of capitalized software costs	[1]			73	72	73
Commonwealth Edison Co [Member]
Capitalized Software [Line Items]
Net unamortized software costs			213	227	213
Amortization of capitalized software costs				73	62	47
PECO Energy Co [Member]
Capitalized Software [Line Items]
Net unamortized software costs			91	111	91
Amortization of capitalized software costs				39	33	33
Baltimore Gas and Electric Company [Member]
Capitalized Software [Line Items]
Net unamortized software costs			164	179	164
Amortization of capitalized software costs				46	44	46
Potomac Electric Power Company [Member]
Capitalized Software [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Net Income					4
Net unamortized software costs			1	2	1
Amortization of capitalized software costs				0	0	(2)
Delmarva Power and Light Company [Member]
Capitalized Software [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Net Income					1
Net unamortized software costs			1	1	1
Amortization of capitalized software costs				0	0	0
Atlantic City Electric Company [Member]
Capitalized Software [Line Items]
Net unamortized software costs			1	1	1
Amortization of capitalized software costs				0	$ 0	0
Successor [Member] | Pepco Holdings LLC [Member]
Capitalized Software [Line Items]
Amortization of capitalized software costs			$ 29	$ 34
Predecessor [Member] | Pepco Holdings LLC [Member]
Capitalized Software [Line Items]
Amortization of capitalized software costs		$ 8				$ 36

[1] Net unamortized software costs Successor Exelon Generation ComEd PECO BGE PHI Pepco DPL ACEDecember 31, 2017$834 $173 $227 $111 $179 $133 $2 $1 $1December 31, 2016808 173 213 91 164 153 1 1 1