Retirement Benefits - Allocated Portion of Pension and Postretirement Benefit Plan Costs (Details) - USD ($) $ in Millions		3 Months Ended	9 Months Ended	12 Months Ended
		Mar. 23, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amount included in capital and operating & maintenance expense				$ 621	$ 637	$ 561
Exelon Generation Co L L C [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amount included in capital and operating & maintenance expense				218	269	250
Commonwealth Edison Co [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amount included in capital and operating & maintenance expense				166	206	162
PECO Energy Co [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amount included in capital and operating & maintenance expense				33	39	36
Baltimore Gas and Electric Company [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amount included in capital and operating & maintenance expense				68	66	67
Pepco Holdings LLC [Member] | Predecessor [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amount included in capital and operating & maintenance expense		$ 23			97	58
Pepco Holdings LLC [Member] | Successor [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amount included in capital and operating & maintenance expense			$ 88
Potomac Electric Power Company [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amount included in capital and operating & maintenance expense		7		31	30	22
Delmarva Power and Light Company [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amount included in capital and operating & maintenance expense		4		18	15	7
Atlantic City Electric Company [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amount included in capital and operating & maintenance expense		3		15	15	13
Corporate, Non-Segment [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amount included in capital and operating & maintenance expense	[1]			48	57	46
Corporate, Non-Segment [Member] | Pepco Holdings LLC [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amount included in capital and operating & maintenance expense		$ 9		$ 47	$ 37	$ 16

[1] ___(a)These amounts primarily represent amounts billed to Exelon’s subsidiaries through intercompany allocations. These amounts are not included in the Generation, ComEd, PECO, BGE, Pepco, DPL or ACE amounts