Supplemental Financial Information - Operations (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018
Decommissioning-Related Activities [Abstract]
Net realized income on NDT funds - Regulatory agreement units	[1],[2]	$ 54	$ 46
Net realized income on NDT funds - Non-regulatory agreement units	[2]	54	56
Net unrealized gains (losses) on NDT funds - Regulatory agreement units	[1]	379	(75)
Net unrealized gains (losses) on NDT funds - Non-regulatory agreement units		280	(96)
Regulatory offset to NDT fund-related activities	[3]	(348)	24
Total decommissioning-related activities		419	(45)
Investment income		12	4
Interest income related to uncertain income tax positions		1	2
AFUDC - Equity		22	18
Non-service net periodic benefit cost		5	(10)
Other		8	3
Other, net		467	(28)
Exelon Generation Co L L C [Member]
Decommissioning-Related Activities [Abstract]
Net realized income on NDT funds - Regulatory agreement units	[1],[2]	54	46
Net realized income on NDT funds - Non-regulatory agreement units	[2]	54	56
Net unrealized gains (losses) on NDT funds - Regulatory agreement units	[1]	379	(75)
Net unrealized gains (losses) on NDT funds - Non-regulatory agreement units		280	(96)
Regulatory offset to NDT fund-related activities	[3]	(348)	24
Total decommissioning-related activities		419	(45)
Investment income		7	2
Interest income related to uncertain income tax positions		0	1
AFUDC - Equity		0	0
Other		4	(2)
Other, net		430	(44)
Commonwealth Edison Co [Member]
Decommissioning-Related Activities [Abstract]
Investment income		0	0
Interest income related to uncertain income tax positions		0	0
AFUDC - Equity		5	6
Other		3	2
Other, net		8	8
PECO Energy Co [Member]
Decommissioning-Related Activities [Abstract]
Investment income		1	0
Interest income related to uncertain income tax positions		0	0
AFUDC - Equity		3	2
Other		0	0
Other, net		4	2
Baltimore Gas and Electric Company [Member]
Decommissioning-Related Activities [Abstract]
Investment income		0	0
Interest income related to uncertain income tax positions		0	0
AFUDC - Equity		5	4
Other		0	0
Other, net		5	4
Pepco Holdings LLC [Member]
Decommissioning-Related Activities [Abstract]
Investment income		0	0
Interest income related to uncertain income tax positions		0	0
AFUDC - Equity		9	6
Other		3	5
Other, net		12	11
Potomac Electric Power Company [Member]
Decommissioning-Related Activities [Abstract]
Investment income		0	0
Interest income related to uncertain income tax positions		0	0
AFUDC - Equity		6	5
Other		1	3
Other, net		7	8
Delmarva Power and Light Company [Member]
Decommissioning-Related Activities [Abstract]
Investment income		0	0
Interest income related to uncertain income tax positions		0	0
AFUDC - Equity		1	1
Other		2	1
Other, net		3	2
Atlantic City Electric Company [Member]
Decommissioning-Related Activities [Abstract]
Investment income		0	0
Interest income related to uncertain income tax positions		0	0
AFUDC - Equity		2	0
Other		1	1
Other, net		$ 3	$ 1

[1] Net realized and unrealized gains (losses) related to Generation’s NDT funds associated with Regulatory Agreement Units are included in Regulatory liabilities in Exelon’s Consolidated Balance Sheets and Noncurrent payables to affiliates in Generation’s Consolidated Balance Sheets.

[2] Realized income includes interest, dividends and realized gains and losses on sales of NDT fund investments.

[3] Includes the elimination of decommissioning-related activities 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 2018 Form 10-K for additional information regarding the accounting for nuclear decommissioning.