Supplemental Financial Information - Operations (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Supplemental Statement of Operations Information [Line Items]
Operating lease income	$ 4	$ 5
Variable lease income	64	69
Decommissioning-Related Activities [Abstract]
Net realized income on NDT funds - Regulatory agreement units	291	47
Net realized income on NDT funds - Non-regulatory agreement units	203	82
Net unrealized gains on NDT funds - Regulatory agreement units	(82)	(932)
Net unrealized gains on NDT funds - Non-regulatory agreement units	(66)	(706)
Regulatory offset to NDT fund-related activities	(167)	709
Decommissioning-related activities	179	(800)
AFUDC - Equity	28	23
Non-service net periodic benefit cost	20	10
Exelon Generation Co L L C [Member]
Supplemental Statement of Operations Information [Line Items]
Operating lease income	3	3
Variable lease income	64	69
Decommissioning-Related Activities [Abstract]
Net realized income on NDT funds - Regulatory agreement units	291	47
Net realized income on NDT funds - Non-regulatory agreement units	203	82
Net unrealized gains on NDT funds - Regulatory agreement units	(82)	(932)
Net unrealized gains on NDT funds - Non-regulatory agreement units	(66)	(706)
Regulatory offset to NDT fund-related activities	(167)	709
Decommissioning-related activities	179	(800)
AFUDC - Equity	0	0
Non-service net periodic benefit cost	0	0
Commonwealth Edison Co [Member]
Decommissioning-Related Activities [Abstract]
Net realized income on NDT funds - Regulatory agreement units	0	0
Net realized income on NDT funds - Non-regulatory agreement units	0	0
Net unrealized gains on NDT funds - Regulatory agreement units	0	0
Net unrealized gains on NDT funds - Non-regulatory agreement units	0	0
Regulatory offset to NDT fund-related activities	0	0
Decommissioning-related activities	0	0
AFUDC - Equity	4	6
Non-service net periodic benefit cost	0	0
PECO Energy Co [Member]
Decommissioning-Related Activities [Abstract]
Net realized income on NDT funds - Regulatory agreement units	0	0
Net realized income on NDT funds - Non-regulatory agreement units	0	0
Net unrealized gains on NDT funds - Regulatory agreement units	0	0
Net unrealized gains on NDT funds - Non-regulatory agreement units	0	0
Regulatory offset to NDT fund-related activities	0	0
Decommissioning-related activities	0	0
AFUDC - Equity	6	3
Non-service net periodic benefit cost	0	0
Baltimore Gas and Electric Company [Member]
Decommissioning-Related Activities [Abstract]
Net realized income on NDT funds - Regulatory agreement units	0	0
Net realized income on NDT funds - Non-regulatory agreement units	0	0
Net unrealized gains on NDT funds - Regulatory agreement units	0	0
Net unrealized gains on NDT funds - Non-regulatory agreement units	0	0
Regulatory offset to NDT fund-related activities	0	0
Decommissioning-related activities	0	0
AFUDC - Equity	7	5
Non-service net periodic benefit cost	0	0
Pepco Holdings LLC [Member]
Supplemental Statement of Operations Information [Line Items]
Operating lease income	1	1
Variable lease income	0	0
Decommissioning-Related Activities [Abstract]
Net realized income on NDT funds - Regulatory agreement units	0	0
Net realized income on NDT funds - Non-regulatory agreement units	0	0
Net unrealized gains on NDT funds - Regulatory agreement units	0	0
Net unrealized gains on NDT funds - Non-regulatory agreement units	0	0
Regulatory offset to NDT fund-related activities	0	0
Decommissioning-related activities	0	0
AFUDC - Equity	11	9
Non-service net periodic benefit cost	0	0
Potomac Electric Power Company [Member]
Decommissioning-Related Activities [Abstract]
Net realized income on NDT funds - Regulatory agreement units	0	0
Net realized income on NDT funds - Non-regulatory agreement units	0	0
Net unrealized gains on NDT funds - Regulatory agreement units	0	0
Net unrealized gains on NDT funds - Non-regulatory agreement units	0	0
Regulatory offset to NDT fund-related activities	0	0
Decommissioning-related activities	0	0
AFUDC - Equity	9	6
Non-service net periodic benefit cost	0	0
Delmarva Power and Light Company [Member]
Supplemental Statement of Operations Information [Line Items]
Operating lease income	1	1
Variable lease income	0	0
Decommissioning-Related Activities [Abstract]
Net realized income on NDT funds - Regulatory agreement units	0	0
Net realized income on NDT funds - Non-regulatory agreement units	0	0
Net unrealized gains on NDT funds - Regulatory agreement units	0	0
Net unrealized gains on NDT funds - Non-regulatory agreement units	0	0
Regulatory offset to NDT fund-related activities	0	0
Decommissioning-related activities	0	0
AFUDC - Equity	1	1
Non-service net periodic benefit cost	0	0
Atlantic City Electric Company [Member]
Decommissioning-Related Activities [Abstract]
Net realized income on NDT funds - Regulatory agreement units	0	0
Net realized income on NDT funds - Non-regulatory agreement units	0	0
Net unrealized gains on NDT funds - Regulatory agreement units	0	0
Net unrealized gains on NDT funds - Non-regulatory agreement units	0	0
Regulatory offset to NDT fund-related activities	0	0
Decommissioning-related activities	0	0
AFUDC - Equity	1	2
Non-service net periodic benefit cost	$ 0	$ 0