Supplemental Financial Information - Operations (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Decommissioning-Related Activities [Abstract]
Net realized income on decommissioning trust funds - Regulatory Agreement Units	[1]	$ 216	$ 211	$ 262	$ 280
Net realized income on decommissioning trust funds - Non-Regulatory Agreement Units	[1]	143	74	199	106
Net unrealized income (losses) on decommissioning trust funds - Regulatory Agreement Units		(194)	(13)	(268)	210
Net unrealized income (losses) on decommissioning trust funds - Non-Regulatory Agreement		(120)	70	(215)	235
Net unrealized income (losses) on pledged assets		4	(2)	2	(2)
Regulatory offset to decommissioning trust fund-related activities	[2]	(23)	(160)	(1)	(396)
Total decommissioning-related activities		26	180	(21)	433
Investment income		6	2	10	4
Unrecognized Tax Benefits Interest Income		2	(1)	4
Income Tax Examination, Penalties and Interest Expense			(1)		(2)
AFUDC - equity		13	17	31	33
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		(11)	(28)	(21)	(54)
Other Income		8	6	14	16
Other, net		44	177	17	434
Exelon Generation Co L L C [Member]
Decommissioning-Related Activities [Abstract]
Net realized income on decommissioning trust funds - Regulatory Agreement Units	[1]	216	211	262	280
Net realized income on decommissioning trust funds - Non-Regulatory Agreement Units	[1]	143	74	199	106
Net unrealized income (losses) on decommissioning trust funds - Regulatory Agreement Units		(194)	(13)	(268)	210
Net unrealized income (losses) on decommissioning trust funds - Non-Regulatory Agreement		(120)	70	(215)	235
Net unrealized income (losses) on pledged assets		4	(2)	2	(2)
Regulatory offset to decommissioning trust fund-related activities	[2]	(23)	(160)	(1)	(396)
Total decommissioning-related activities		26	180	(21)	433
Investment income		5	1	7	3
Unrecognized Tax Benefits Interest Income		0	0	1
Income Tax Examination, Penalties and Interest Expense			0		0
AFUDC - equity		0	0	0	0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		0	0	0	0
Other Income		(2)	0	(2)	4
Other, net		29	181	(15)	440
Commonwealth Edison Co [Member]
Decommissioning-Related Activities [Abstract]
Net realized income on decommissioning trust funds - Regulatory Agreement Units	[1]	0	0	0	0
Net realized income on decommissioning trust funds - Non-Regulatory Agreement Units	[1]	0	0	0	0
Net unrealized income (losses) on decommissioning trust funds - Regulatory Agreement Units		0	0	0	0
Net unrealized income (losses) on decommissioning trust funds - Non-Regulatory Agreement		0	0	0	0
Net unrealized income (losses) on pledged assets		0	0	0	0
Regulatory offset to decommissioning trust fund-related activities	[2]	0	0	0	0
Total decommissioning-related activities		0	0	0	0
Investment income		0	0	0	0
Unrecognized Tax Benefits Interest Income		0	0	0
Income Tax Examination, Penalties and Interest Expense			0		0
AFUDC - equity		2	2	8	4
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		0	0	0	0
Other Income		2	2	4	4
Other, net		4	4	12	8
PECO Energy Co [Member]
Decommissioning-Related Activities [Abstract]
Net realized income on decommissioning trust funds - Regulatory Agreement Units	[1]	0	0	0	0
Net realized income on decommissioning trust funds - Non-Regulatory Agreement Units	[1]	0	0	0	0
Net unrealized income (losses) on decommissioning trust funds - Regulatory Agreement Units		0	0	0	0
Net unrealized income (losses) on decommissioning trust funds - Non-Regulatory Agreement		0	0	0	0
Net unrealized income (losses) on pledged assets		0	0	0	0
Regulatory offset to decommissioning trust fund-related activities	[2]	0	0	0	0
Total decommissioning-related activities		0	0	0	0
Investment income		0	0	0	(1)
Unrecognized Tax Benefits Interest Income		0	0	0
Income Tax Examination, Penalties and Interest Expense			0		0
AFUDC - equity		0	2	2	4
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		0	0	0	0
Other Income		0	0	0	0
Other, net		0	2	2	3
Baltimore Gas and Electric Company [Member]
Decommissioning-Related Activities [Abstract]
Net realized income on decommissioning trust funds - Regulatory Agreement Units	[1]	0	0	0	0
Net realized income on decommissioning trust funds - Non-Regulatory Agreement Units	[1]	0	0	0	0
Net unrealized income (losses) on decommissioning trust funds - Regulatory Agreement Units		0	0	0	0
Net unrealized income (losses) on decommissioning trust funds - Non-Regulatory Agreement		0	0	0	0
Net unrealized income (losses) on pledged assets		0	0	0	0
Regulatory offset to decommissioning trust fund-related activities	[2]	0	0	0	0
Total decommissioning-related activities		0	0	0	0
Investment income		0	0	0	0
Unrecognized Tax Benefits Interest Income		0	0	0
Income Tax Examination, Penalties and Interest Expense			0		0
AFUDC - equity		4	4	8	8
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		0	0	0	0
Other Income		0	0	1	0
Other, net		4	4	9	8
Pepco Holdings LLC [Member]
Decommissioning-Related Activities [Abstract]
Net realized income on decommissioning trust funds - Regulatory Agreement Units	[1]	0	0	0	0
Net realized income on decommissioning trust funds - Non-Regulatory Agreement Units	[1]	0	0	0	0
Net unrealized income (losses) on decommissioning trust funds - Regulatory Agreement Units		0	0	0	0
Net unrealized income (losses) on decommissioning trust funds - Non-Regulatory Agreement		0	0	0	0
Net unrealized income (losses) on pledged assets		0	0	0	0
Regulatory offset to decommissioning trust fund-related activities	[2]	0	0	0	0
Total decommissioning-related activities		0	0	0	0
Investment income		0	0	0	1
Unrecognized Tax Benefits Interest Income		0	0	0
Income Tax Examination, Penalties and Interest Expense			0		0
AFUDC - equity		7	9	13	17
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		0	0	0	0
Other Income		4	4	9	8
Other, net		11	13	22	26
Potomac Electric Power Company [Member]
Decommissioning-Related Activities [Abstract]
Net realized income on decommissioning trust funds - Regulatory Agreement Units	[1]	0	0	0	0
Net realized income on decommissioning trust funds - Non-Regulatory Agreement Units	[1]	0	0	0	0
Net unrealized income (losses) on decommissioning trust funds - Regulatory Agreement Units		0	0	0	0
Net unrealized income (losses) on decommissioning trust funds - Non-Regulatory Agreement		0	0	0	0
Net unrealized income (losses) on pledged assets		0	0	0	0
Regulatory offset to decommissioning trust fund-related activities	[2]	0	0	0	0
Total decommissioning-related activities		0	0	0	0
Investment income		0	0	0	1
Unrecognized Tax Benefits Interest Income		0	0	0
Income Tax Examination, Penalties and Interest Expense			0		0
AFUDC - equity		6	5	12	11
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		0	0	0	0
Other Income		2	2	4	3
Other, net		8	7	16	15
Delmarva Power and Light Company [Member]
Decommissioning-Related Activities [Abstract]
Net realized income on decommissioning trust funds - Regulatory Agreement Units	[1]	0	0	0	0
Net realized income on decommissioning trust funds - Non-Regulatory Agreement Units	[1]	0	0	0	0
Net unrealized income (losses) on decommissioning trust funds - Regulatory Agreement Units		0	0	0	0
Net unrealized income (losses) on decommissioning trust funds - Non-Regulatory Agreement		0	0	0	0
Net unrealized income (losses) on pledged assets		0	0	0	0
Regulatory offset to decommissioning trust fund-related activities	[2]	0	0	0	0
Total decommissioning-related activities		0	0	0	0
Investment income		0	0	0	0
Unrecognized Tax Benefits Interest Income		0	0	0
Income Tax Examination, Penalties and Interest Expense			0		0
AFUDC - equity		1	2	1	3
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		0	0	0	0
Other Income		2	1	4	3
Other, net		3	3	5	6
Atlantic City Electric Company [Member]
Decommissioning-Related Activities [Abstract]
Net realized income on decommissioning trust funds - Regulatory Agreement Units	[1]	0	0	0	0
Net realized income on decommissioning trust funds - Non-Regulatory Agreement Units	[1]	0	0	0	0
Net unrealized income (losses) on decommissioning trust funds - Regulatory Agreement Units		0	0	0	0
Net unrealized income (losses) on decommissioning trust funds - Non-Regulatory Agreement		0	0	0	0
Net unrealized income (losses) on pledged assets		0	0	0	0
Regulatory offset to decommissioning trust fund-related activities	[2]	0	0	0	0
Total decommissioning-related activities		0	0	0	0
Investment income		0	0	0	0
Unrecognized Tax Benefits Interest Income		0	0	0
Income Tax Examination, Penalties and Interest Expense			0		0
AFUDC - equity		0	2	0	3
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		0	0	0	0
Other Income		1	0	1	1
Other, net		$ 1	$ 2	$ 1	$ 4

[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 15 — Asset Retirement Obligations of the Exelon 2017 Form 10-K for additional information regarding the accounting for nuclear decommissioning.