Supplemental Financial Information - Operations (Detail) - 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
Decommissioning-Related Activities [Abstract]
Net realized income on decommissioning trust funds - Regulatory Agreement Units	$ 211	$ 90	[1]			$ 280		$ 122	[1]
Net realized income on decommissioning trust funds - Non-Regulatory Agreement Units	74	39	[1]			106		61	[1]
Net unrealized income (losses) on decommissioning trust funds - Regulatory Agreement Units	(13)	52				210		131
Net unrealized income (losses) on decommissioning trust funds - Non-Regulatory Agreement	70	48				235		100
Net unrealized income (losses) on pledged assets	(2)	1				(2)		3
Regulatory offset to decommissioning trust fund-related activities	(160)	(117)	[2]			(396)		(211)	[2]
Total decommissioning-related activities	180	113				433		206
Investment income	2	6				4		12
Long-term lease income								4
Unrecognized Tax Benefits Interest Income	(1)	(4)						(5)
Income Tax Examination, Penalties and Interest Expense	1					2
AFUDC - equity	17	15				33		24
Gains (Losses) on Extinguishment of Debt								(3)
Other Income	6	6				16		10
Other, net	205	144				488		258
Exelon Generation Co L L C [Member]
Decommissioning-Related Activities [Abstract]
Net realized income on decommissioning trust funds - Regulatory Agreement Units	211	90	[1]			280	[1]	122	[1]
Net realized income on decommissioning trust funds - Non-Regulatory Agreement Units	74	39	[1]			106	[1]	61	[1]
Net unrealized income (losses) on decommissioning trust funds - Regulatory Agreement Units	(13)	52				210		131
Net unrealized income (losses) on decommissioning trust funds - Non-Regulatory Agreement	70	48				235		100
Net unrealized income (losses) on pledged assets	(2)	1				(2)		3
Regulatory offset to decommissioning trust fund-related activities	(160)	(117)	[2]			(396)	[2]	(211)	[2]
Total decommissioning-related activities	180	113				433		206
Investment income	1	5				3		5
Unrecognized Tax Benefits Interest Income	0	0						0
Gains (Losses) on Extinguishment of Debt								(2)
Other Income	0	(1)				4		1
Other, net	181	117				440		210
Commonwealth Edison Co [Member]
Decommissioning-Related Activities [Abstract]
Investment income								0
Unrecognized Tax Benefits Interest Income								0
Income Tax Examination, Penalties and Interest Expense	0					0
AFUDC - equity	2	1				4		3
Other Income	2	2				4		4
Other, net	4	3				8		7
PECO Energy Co [Member]
Decommissioning-Related Activities [Abstract]
Investment income						(1)		(1)
Unrecognized Tax Benefits Interest Income								0
AFUDC - equity	2	2				4		4
Other Income	0	0				0		1
Other, net	2	2				3		4
Baltimore Gas and Electric Company [Member]
Decommissioning-Related Activities [Abstract]
Investment income								2
Unrecognized Tax Benefits Interest Income								0
AFUDC - equity	4	5				8		9
Other Income	0	0				0		0
Other, net	4	5				8		11
Pepco Holdings LLC [Member] | Successor [Member]
Decommissioning-Related Activities [Abstract]
Investment income						1
Unrecognized Tax Benefits Interest Income				$ 0
AFUDC - equity	9	7		8		17
Other Income	4	4		4		8
Other, net	13	11		$ 12		26
Pepco Holdings LLC [Member] | Predecessor [Member]
Decommissioning-Related Activities [Abstract]
Unrecognized Tax Benefits Interest Income					$ 0
AFUDC - equity					7
Other Income					(11)
Other, net					$ (4)
Potomac Electric Power Company [Member]
Decommissioning-Related Activities [Abstract]
Investment income						1
Unrecognized Tax Benefits Interest Income								(1)
AFUDC - equity	5	5				11		9
Other Income	2	1				3		4
Other, net	7	6				15		14
Delmarva Power and Light Company [Member]
Decommissioning-Related Activities [Abstract]
Unrecognized Tax Benefits Interest Income								0
AFUDC - equity	2	1				3		2
Other Income	1	2				3		4
Other, net	3	3				6		6
Atlantic City Electric Company [Member]
Decommissioning-Related Activities [Abstract]
Unrecognized Tax Benefits Interest Income								(1)
AFUDC - equity	2	1				3		3
Other Income	0	1				1		1
Other, net	$ 2	$ 2				$ 4		$ 5

[1] Includes investment income and realized gains and losses on sales of investments of the trust funds.

[2] Includes the elimination of NDT fund activity for the Regulatory Agreement Units, including the elimination of net income taxes related to all NDT fund activity for those units. See Note 16 — Asset Retirement Obligations of the Exelon 2016 Form 10-K for additional information regarding the accounting for nuclear decommissioning.