Retirement Benefits - Defined Contribution Savings Plans (Details) - USD ($) $ in Millions		3 Months Ended			9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017		Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions	[1]	$ 44	$ 34	[2]	$ 126	$ 97	[2]
Corporate, Non-Segment [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions	[3]	5	2		16	7
Exelon Generation Co L L C [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions		23	14	[2]	65	42	[2]
Commonwealth Edison Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions		8	9		23	24
PECO Energy Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions		2	3		7	7
Baltimore Gas and Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions		2	3		5	7
Potomac Electric Power Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions		1	1		2	3
Delmarva Power and Light Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions		1	1		2	2
Atlantic City Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions		1	0		2	1
Pepco Holdings LLC [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions	[1]	4	3		10	10
Pepco Holdings LLC [Member] | Corporate, Non-Segment [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions	[4]	$ 1	$ 1		$ 4	$ 4

[1] Exelon reflects the consolidated savings plan matching contributions of Generation, ComEd, PECO, BGE, BSC, and PHI. PHI reflects the consolidated savings plan matching contributions of Pepco, DPL, ACE, and PHISCO.

[2] FitzPatrick net benefit costs are included for the period after the acquisition date of March 31, 2017.

[3] These amounts primarily represent amounts billed to Exelon’s subsidiaries through intercompany allocations. These amounts are not included in the Generation, ComEd, PECO, BGE, PHI, Pepco, DPL, ACE or PHISCO amounts above.

[4] These amounts represent amounts billed to Pepco and DPL through intercompany allocations. These amounts are not included in Pepco or DPL amounts above.