Retirement Benefits - Defined Contribution Savings Plans (Details) - USD ($) $ in Millions		3 Months Ended						6 Months Ended
		Jun. 30, 2017		Jun. 30, 2016		Jun. 30, 2016	Mar. 23, 2016	Jun. 30, 2017		Jun. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions		$ 33		$ 30				$ 63		$ 56
Corporate, Non-Segment [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions	[1]	3		2				5		7
Exelon Generation Co L L C [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions		14		13				28		25
Commonwealth Edison Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions		8		7				15		13
PECO Energy Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions		2		2				4		4
Baltimore Gas and Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions		3		2				4		3
Potomac Electric Power Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions		1	[2]	1	[2]		$ 1	2	[2]	2	[2]
Delmarva Power and Light Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions		1	[2]	0	[2]		1	1	[2]	1	[2]
Atlantic City Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions	[2]	0		1				1		1
Pepco Holdings LLC [Member] | Successor [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions		3						7
Pepco Holdings LLC [Member] | Predecessor [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions				4		$ 4	3
Pepco Holdings LLC [Member] | Corporate, Non-Segment [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions	[1],[2]	$ 1		$ 2			$ 1	$ 3		$ 3

[1] These amounts primarily represent amounts billed to Exelon and PHI's subsidiaries through intercompany allocations. These costs are not included in the Generation, ComEd, PECO, BGE, Pepco and DPL amounts above.

[2] Pepco's, DPL's and PHISCO's matching contributions for the six months ended June 30, 2016 include $1 million, $1 million and $1 million, respectively, of costs incurred prior to the closing of Exelon’s merger with PHI on March 23, 2016, which is not included in Exelon’s matching contributions for the six months ended June 30, 2016.