Supplemental Financial Information - Operations (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Decommissioning-Related Activities [Abstract]
Net realized income on decommissioning trust funds - Regulatory Agreement Units	[1]	$ 214	$ 159	$ 476	$ 439
Net realized income on decommissioning trust funds - Non-Regulatory Agreement Units	[1]	58	59	257	165
Net unrealized income (losses) on decommissioning trust funds - Regulatory Agreement Units		(66)	44	(335)	253
Net unrealized income (losses) on decommissioning trust funds - Non-Regulatory Agreement		72	111	(143)	347
Net unrealized income (losses) on pledged assets		(9)	(4)	(7)	(5)
Regulatory offset to decommissioning trust fund-related activities	[2]	(110)	(161)	(110)	(558)
Total decommissioning-related activities		159	208	138	641
Investment income		9	2	19	6
Unrecognized Tax Benefits Interest Income		1	4	5	3
Penalty income related to uncertain income tax positions					2
AFUDC - equity		16	17	47	51
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		(12)	(27)	(33)	(82)
Other Income		21	6	36	22
Other, net		194	210	212	643
Exelon Generation Co L L C [Member]
Decommissioning-Related Activities [Abstract]
Net realized income on decommissioning trust funds - Regulatory Agreement Units	[1]	214	159	476	439
Net realized income on decommissioning trust funds - Non-Regulatory Agreement Units	[1]	58	59	257	165
Net unrealized income (losses) on decommissioning trust funds - Regulatory Agreement Units		(66)	44	(335)	253
Net unrealized income (losses) on decommissioning trust funds - Non-Regulatory Agreement		72	111	(143)	347
Net unrealized income (losses) on pledged assets		(9)	(4)	(7)	(5)
Regulatory offset to decommissioning trust fund-related activities	[2]	(110)	(161)	(110)	(558)
Total decommissioning-related activities		159	208	138	641
Investment income		5	1	12	4
Unrecognized Tax Benefits Interest Income		0	0	1	0
Penalty income related to uncertain income tax positions					0
AFUDC - equity		0	0	0	0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		0	0	0	0
Other Income		15	0	13	3
Other, net		179	209	164	648
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	0
Penalty income related to uncertain income tax positions					0
AFUDC - equity		4	2	12	6
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		0	0	0	0
Other Income		3	3	9	8
Other, net		7	5	21	14
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	0
Unrecognized Tax Benefits Interest Income		0	0	0	0
Penalty income related to uncertain income tax positions					0
AFUDC - equity		1	2	3	6
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		0	0	0	0
Other Income		1	0	1	0
Other, net		2	2	4	6
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	0
Penalty income related to uncertain income tax positions					0
AFUDC - equity		5	4	13	12
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		0	0	0	0
Other Income		0	0	1	0
Other, net		5	4	14	12
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		2	1	3	2
Unrecognized Tax Benefits Interest Income		0	0	0	0
Penalty income related to uncertain income tax positions					0
AFUDC - equity		6	9	19	27
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		0	0	0	0
Other Income		3	3	11	11
Other, net		11	13	33	40
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		1	1	1	1
Unrecognized Tax Benefits Interest Income		0	0	0	0
Penalty income related to uncertain income tax positions					0
AFUDC - equity		6	6	17	17
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		0	0	0	0
Other Income		0	0	5	4
Other, net		7	7	23	22
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		1	0	1	0
Unrecognized Tax Benefits Interest Income		0	0	0	0
Penalty income related to uncertain income tax positions					0
AFUDC - equity		0	2	2	5
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		0	0	0	0
Other Income		1	2	4	5
Other, net		2	4	7	10
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	0
Penalty income related to uncertain income tax positions					0
AFUDC - equity		0	1	0	5
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		0	0	0	0
Other Income		1	0	2	1
Other, net		$ 1	$ 1	$ 2	$ 6

[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.