Supplemental Financial Information - Operations (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Supplemental Statement of Operations Information [Line Items]
Operating Lease Income	$ 14	$ 14	$ 18	$ 18
Variable Lease Income	80	77	149	129
Decommissioning-Related Activities [Abstract]
Net realized income on NDT funds - Regulatory agreement units	30	77	77	131
Net realized income on NDT funds - Non-regulatory agreement units	23	230	104	283
Net unrealized (losses) gains on NDT funds - Regulatory agreement units	645	98	(287)	476
Net unrealized (losses) gains on NDT funds - Non-regulatory agreement units	452	(98)	(253)	182
Regulatory offset to NDT fund-related activities	(542)	(141)	167	(487)
Decommissioning-related activities	608	166	(192)	585
AFUDC - Equity	26	21	49	43
Non-service net periodic benefit cost	14	5	24	10
Exelon Generation Co L L C [Member]
Supplemental Statement of Operations Information [Line Items]
Operating Lease Income	12	12	15	15
Variable Lease Income	79	74	148	126
Decommissioning-Related Activities [Abstract]
Net realized income on NDT funds - Regulatory agreement units	30	77	77	131
Net realized income on NDT funds - Non-regulatory agreement units	23	230	104	283
Net unrealized (losses) gains on NDT funds - Regulatory agreement units	645	98	(287)	476
Net unrealized (losses) gains on NDT funds - Non-regulatory agreement units	452	(98)	(253)	182
Regulatory offset to NDT fund-related activities	(542)	(141)	167	(487)
Decommissioning-related activities	608	166	(192)	585
AFUDC - Equity	0	0	0	0
Commonwealth Edison Co [Member]
Supplemental Statement of Operations Information [Line Items]
Operating Lease Income	0	0	0	0
Variable Lease Income	0	0	0	0
Decommissioning-Related Activities [Abstract]
AFUDC - Equity	8	4	14	9
PECO Energy Co [Member]
Supplemental Statement of Operations Information [Line Items]
Operating Lease Income	0	0	0	0
Variable Lease Income	0	0	0	0
Decommissioning-Related Activities [Abstract]
AFUDC - Equity	4	3	7	6
Baltimore Gas and Electric Company [Member]
Supplemental Statement of Operations Information [Line Items]
Operating Lease Income	0	0	0	0
Variable Lease Income	0	0	0	0
Decommissioning-Related Activities [Abstract]
AFUDC - Equity	6	5	10	10
Pepco Holdings LLC [Member]
Supplemental Statement of Operations Information [Line Items]
Operating Lease Income	1	1	2	2
Variable Lease Income	1	3	1	3
Decommissioning-Related Activities [Abstract]
AFUDC - Equity	8	9	17	18
Potomac Electric Power Company [Member]
Supplemental Statement of Operations Information [Line Items]
Operating Lease Income	0	0	0	0
Variable Lease Income	0	0	0	0
Decommissioning-Related Activities [Abstract]
AFUDC - Equity	6	6	13	12
Delmarva Power and Light Company [Member]
Supplemental Statement of Operations Information [Line Items]
Operating Lease Income	1	1	2	2
Variable Lease Income	1	3	1	3
Decommissioning-Related Activities [Abstract]
AFUDC - Equity	1	1	2	2
Atlantic City Electric Company [Member]
Supplemental Statement of Operations Information [Line Items]
Operating Lease Income	0	0	0	0
Variable Lease Income	0	0	0	0
Decommissioning-Related Activities [Abstract]
AFUDC - Equity	$ 1	$ 2	$ 2	$ 4