Supplemental Financial Information - Operations (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes		$ 235	$ 224
Decommissioning-Related Activities [Abstract]
Net realized income on decommissioning trust funds - Regulatory Agreement Units		46	68	[1]
Net realized income on decommissioning trust funds - Non-Regulatory Agreement Units		56	32	[1]
Net unrealized income (losses) on decommissioning trust funds - Regulatory Agreement Units		(75)	222
Net unrealized income (losses) on decommissioning trust funds - Non-Regulatory Agreement		(96)	166
Net unrealized income (losses) on pledged assets		(2)	(1)
Regulatory offset to decommissioning trust fund-related activities		24	(234)	[2]
Total decommissioning-related activities		47	(253)
Investment income		4	2
Unrecognized Tax Benefits Interest Income		2	1
AFUDC - equity		18	17
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		10	26
Other Income		5	10
Other, net		(28)	257
Exelon Generation Co L L C [Member]
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes		32	32
Decommissioning-Related Activities [Abstract]
Net realized income on decommissioning trust funds - Regulatory Agreement Units	[1]	46	68
Net realized income on decommissioning trust funds - Non-Regulatory Agreement Units	[1]	56	32
Net unrealized income (losses) on decommissioning trust funds - Regulatory Agreement Units		(75)	222
Net unrealized income (losses) on decommissioning trust funds - Non-Regulatory Agreement		(96)	166
Net unrealized income (losses) on pledged assets		(2)	(1)
Regulatory offset to decommissioning trust fund-related activities	[2]	24	(234)
Total decommissioning-related activities		47	(253)
Investment income		2	2
Unrecognized Tax Benefits Interest Income		1	0
AFUDC - equity		0	0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		0	0
Other Income		0	4
Other, net		(44)	259
Commonwealth Edison Co [Member]
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes		61	59
Decommissioning-Related Activities [Abstract]
Net realized income on decommissioning trust funds - Regulatory Agreement Units		0	0	[1]
Net realized income on decommissioning trust funds - Non-Regulatory Agreement Units		0	0	[1]
Net unrealized income (losses) on decommissioning trust funds - Regulatory Agreement Units		0	0
Net unrealized income (losses) on decommissioning trust funds - Non-Regulatory Agreement		0	0
Net unrealized income (losses) on pledged assets		0	0
Regulatory offset to decommissioning trust fund-related activities		0	0	[2]
Total decommissioning-related activities		0	0
Investment income		0	0
Unrecognized Tax Benefits Interest Income		0	0
AFUDC - equity		6	2
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		0	0
Other Income		2	2
Other, net		8	4
PECO Energy Co [Member]
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes		33	31
Decommissioning-Related Activities [Abstract]
Net realized income on decommissioning trust funds - Regulatory Agreement Units		0	0	[1]
Net realized income on decommissioning trust funds - Non-Regulatory Agreement Units		0	0	[1]
Net unrealized income (losses) on decommissioning trust funds - Regulatory Agreement Units		0	0
Net unrealized income (losses) on decommissioning trust funds - Non-Regulatory Agreement		0	0
Net unrealized income (losses) on pledged assets		0	0
Regulatory offset to decommissioning trust fund-related activities		0	0	[2]
Total decommissioning-related activities		0	0
Investment income		0	0
Unrecognized Tax Benefits Interest Income		0	0
AFUDC - equity		2	2
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		0	0
Other Income		0	0
Other, net		2	2
Baltimore Gas and Electric Company [Member]
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes		26	26
Decommissioning-Related Activities [Abstract]
Net realized income on decommissioning trust funds - Regulatory Agreement Units		0	0	[1]
Net realized income on decommissioning trust funds - Non-Regulatory Agreement Units		0	0	[1]
Net unrealized income (losses) on decommissioning trust funds - Regulatory Agreement Units		0	0
Net unrealized income (losses) on decommissioning trust funds - Non-Regulatory Agreement		0	0
Net unrealized income (losses) on pledged assets		0	0
Regulatory offset to decommissioning trust fund-related activities		0	0	[2]
Total decommissioning-related activities		0	0
Investment income		0	0
Unrecognized Tax Benefits Interest Income		0	0
AFUDC - equity		4	4
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		0	0
Other Income		0	0
Other, net		4	4
Pepco Holdings LLC [Member]
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes		83	76
Decommissioning-Related Activities [Abstract]
Net realized income on decommissioning trust funds - Regulatory Agreement Units		0	0	[1]
Net realized income on decommissioning trust funds - Non-Regulatory Agreement Units		0	0	[1]
Net unrealized income (losses) on decommissioning trust funds - Regulatory Agreement Units		0	0
Net unrealized income (losses) on decommissioning trust funds - Non-Regulatory Agreement		0	0
Net unrealized income (losses) on pledged assets		0	0
Regulatory offset to decommissioning trust fund-related activities		0	0	[2]
Total decommissioning-related activities		0	0
Investment income		0	0
Unrecognized Tax Benefits Interest Income		0	0
AFUDC - equity		6	9
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		0	0
Other Income		5	4
Other, net		11	13
Potomac Electric Power Company [Member]
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes		77	71
Decommissioning-Related Activities [Abstract]
Net realized income on decommissioning trust funds - Regulatory Agreement Units		0	0	[1]
Net realized income on decommissioning trust funds - Non-Regulatory Agreement Units		0	0	[1]
Net unrealized income (losses) on decommissioning trust funds - Regulatory Agreement Units		0	0
Net unrealized income (losses) on decommissioning trust funds - Non-Regulatory Agreement		0	0
Net unrealized income (losses) on pledged assets		0	0
Regulatory offset to decommissioning trust fund-related activities		0	0	[2]
Total decommissioning-related activities		0	0
Investment income		0	0
Unrecognized Tax Benefits Interest Income		0	0
AFUDC - equity		5	5
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		0	0
Other Income		3	3
Other, net		8	8
Delmarva Power and Light Company [Member]
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes		6	5
Decommissioning-Related Activities [Abstract]
Net realized income on decommissioning trust funds - Regulatory Agreement Units		0	0	[1]
Net realized income on decommissioning trust funds - Non-Regulatory Agreement Units		0	0	[1]
Net unrealized income (losses) on decommissioning trust funds - Regulatory Agreement Units		0	0
Net unrealized income (losses) on decommissioning trust funds - Non-Regulatory Agreement		0	0
Net unrealized income (losses) on pledged assets		0	0
Regulatory offset to decommissioning trust fund-related activities		0	0	[2]
Total decommissioning-related activities		0	0
Investment income		0	0
Unrecognized Tax Benefits Interest Income		0	0
AFUDC - equity		1	2
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		0	0
Other Income		1	1
Other, net		2	3
Atlantic City Electric Company [Member]
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes		0	0
Decommissioning-Related Activities [Abstract]
Net realized income on decommissioning trust funds - Regulatory Agreement Units		0	0	[1]
Net realized income on decommissioning trust funds - Non-Regulatory Agreement Units		0	0	[1]
Net unrealized income (losses) on decommissioning trust funds - Regulatory Agreement Units		0	0
Net unrealized income (losses) on decommissioning trust funds - Non-Regulatory Agreement		0	0
Net unrealized income (losses) on pledged assets		0	0
Regulatory offset to decommissioning trust fund-related activities		0	0	[2]
Total decommissioning-related activities		0	0
Investment income		0	0
Unrecognized Tax Benefits Interest Income		0	0
AFUDC - equity		0	2
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		0	0
Other Income		1	0
Other, net		$ 1	$ 2

[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.