Supplemental Financial Information - Operations (Detail) - USD ($) $ in Millions	3 Months Ended				6 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016		Mar. 23, 2016	Sep. 30, 2016	Sep. 30, 2017		Sep. 30, 2016
Decommissioning-Related Activities [Abstract]
Net realized income on decommissioning trust funds - Regulatory Agreement Units	$ 159	$ 57	[1]			$ 439		$ 181	[1]
Net realized income on decommissioning trust funds - Non-Regulatory Agreement Units	59	35	[1]			165		95	[1]
Net unrealized income (losses) on decommissioning trust funds - Regulatory Agreement Units	44	155				253		286
Net unrealized income (losses) on decommissioning trust funds - Non-Regulatory Agreement	111	116				347		216
Net unrealized income (losses) on pledged assets	(4)	(5)				(5)		(2)
Regulatory offset to decommissioning trust fund-related activities	(161)	(168)	[2]			(558)		(380)	[2]
Total decommissioning-related activities	208	190				641		396
Investment income	2	2				6		14
Long-term lease income								4
Unrecognized Tax Benefits Interest Income	(4)	(8)				(3)		(13)
Income Tax Examination, Penalties and Interest Expense		106				(2)		106
AFUDC - equity	17	19				51		43
Gains (Losses) on Extinguishment of Debt								(3)
Other Income	6	7				22		16
Other, net	237	120				725		377
Exelon Generation Co L L C [Member]
Decommissioning-Related Activities [Abstract]
Net realized income on decommissioning trust funds - Regulatory Agreement Units	159	57	[1]			439	[1]	181	[1]
Net realized income on decommissioning trust funds - Non-Regulatory Agreement Units	59	35	[1]			165	[1]	95	[1]
Net unrealized income (losses) on decommissioning trust funds - Regulatory Agreement Units	44	155				253		286
Net unrealized income (losses) on decommissioning trust funds - Non-Regulatory Agreement	111	116				347		216
Net unrealized income (losses) on pledged assets	(4)	(5)				(5)		(2)
Regulatory offset to decommissioning trust fund-related activities	(161)	(168)	[2]			(558)	[2]	(380)	[2]
Total decommissioning-related activities	208	190				641		396
Investment income	1	1				4		6
Long-term lease income								0
Unrecognized Tax Benefits Interest Income	0	0				0		0
Income Tax Examination, Penalties and Interest Expense		0				0		0
Gains (Losses) on Extinguishment of Debt								(2)
Other Income	0	(6)				3		(5)
Other, net	209	185				648		395
Commonwealth Edison Co [Member]
Decommissioning-Related Activities [Abstract]
Investment income	0	0				0		0
Long-term lease income								0
Unrecognized Tax Benefits Interest Income	0	0				0		0
Income Tax Examination, Penalties and Interest Expense		(86)				0		86
AFUDC - equity	2	5				6		8
Other Income	3	1				8		6
Other, net	5	(80)				14		(72)
PECO Energy Co [Member]
Decommissioning-Related Activities [Abstract]
Investment income	0	(1)				0		(1)
Long-term lease income								0
Unrecognized Tax Benefits Interest Income	0	0				0		0
Income Tax Examination, Penalties and Interest Expense		0				0
AFUDC - equity	2	2				6		6
Other Income	0	1				0		1
Other, net	2	2				6		6
Baltimore Gas and Electric Company [Member]
Decommissioning-Related Activities [Abstract]
Investment income	0	0				0		2
Long-term lease income								0
Unrecognized Tax Benefits Interest Income	0	0				0		0
Income Tax Examination, Penalties and Interest Expense		0				0
AFUDC - equity	4	5				12		14
Other Income	0	0				0		0
Other, net	4	5				12		16
Pepco Holdings LLC [Member] | Successor [Member]
Decommissioning-Related Activities [Abstract]
Investment income		0			$ 1	2
Long-term lease income					0
Unrecognized Tax Benefits Interest Income	0	0			0	0
Income Tax Examination, Penalties and Interest Expense		0				0
AFUDC - equity	9				15	27
Other Income	3				15	11
Other, net	13	19			$ 31	40
Pepco Holdings LLC [Member] | Predecessor [Member]
Decommissioning-Related Activities [Abstract]
Investment income				$ 0
Long-term lease income				0
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	0				1		0
Long-term lease income								0
Unrecognized Tax Benefits Interest Income	0	0				0		1
Income Tax Examination, Penalties and Interest Expense		0				0
AFUDC - equity	6	5				17		14
Other Income	0	7				4		13
Other, net	7	12				22		28
Delmarva Power and Light Company [Member]
Decommissioning-Related Activities [Abstract]
Investment income	0	0				0		0
Long-term lease income								0
Unrecognized Tax Benefits Interest Income	0	0				0		0
Income Tax Examination, Penalties and Interest Expense		0				0
AFUDC - equity	2	1				5		3
Other Income	2	2				5		6
Other, net	4	3				10		9
Atlantic City Electric Company [Member]
Decommissioning-Related Activities [Abstract]
Investment income	0	0				0		0
Long-term lease income								0
Unrecognized Tax Benefits Interest Income	0	0				0		1
Income Tax Examination, Penalties and Interest Expense		0				0
AFUDC - equity	1	1				5		5
Other Income	0	1				1		2
Other, net	$ 1	$ 2				$ 6		$ 8

[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.