Supplemental Financial Information - Balance Sheet (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	$ 1,858	$ 587	$ 1,683	$ 1,349
Restricted cash	485	358	309	247
Restricted cash included in other long-term assets	137	177	186	185
Total cash, cash equivalents and restricted cash	2,480	1,122	2,178	1,781
Accrued Expenses [Abstract]
Compensation-related accruals	898	1,052
Taxes accrued	403	414
Interest accrued	440	337
Exelon Generation Co L L C [Member]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	623	303	1,019	750
Restricted cash	100	146	126	153
Restricted cash included in other long-term assets	0	0	0	0
Total cash, cash equivalents and restricted cash	723	449	1,145	903
Accrued Expenses [Abstract]
Compensation-related accruals	352	422
Taxes accrued	183	222
Interest accrued	79	65
Commonwealth Edison Co [Member]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	76	90	76	135
Restricted cash	305	150	124	29
Restricted cash included in other long-term assets	127	163	171	166
Total cash, cash equivalents and restricted cash	508	403	371	330
Accrued Expenses [Abstract]
Compensation-related accruals	147	171
Taxes accrued	55	83
Interest accrued	64	110
PECO Energy Co [Member]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	242	21	224	130
Restricted cash	7	6	6	5
Restricted cash included in other long-term assets	0	0	0	0
Total cash, cash equivalents and restricted cash	249	27	230	135
Accrued Expenses [Abstract]
Compensation-related accruals	60	58
Taxes accrued	25	3
Interest accrued	36	37
Baltimore Gas and Electric Company [Member]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	326	24	130	7
Restricted cash	1	1	1	6
Restricted cash included in other long-term assets	0	0	0	0
Total cash, cash equivalents and restricted cash	327	25	131	13
Accrued Expenses [Abstract]
Compensation-related accruals	72	78
Taxes accrued	63	26
Interest accrued	40	46
Pepco Holdings LLC [Member]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	196	131	99	124
Restricted cash	38	36	38	43
Restricted cash included in other long-term assets	10	14	15	19
Total cash, cash equivalents and restricted cash	244	181	152	186
Accrued Expenses [Abstract]
Compensation-related accruals	95	101
Taxes accrued	84	117
Interest accrued	80	49
Potomac Electric Power Company [Member]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	125	30	18	16
Restricted cash	33	33	34	37
Restricted cash included in other long-term assets	0	0	0	0
Total cash, cash equivalents and restricted cash	158	63	52	53
Accrued Expenses [Abstract]
Compensation-related accruals	31	28
Taxes accrued	65	90
Interest accrued	38	23
Delmarva Power and Light Company [Member]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	26	13	11	23
Restricted cash	0	0	0	1
Restricted cash included in other long-term assets	0	0	0	0
Total cash, cash equivalents and restricted cash	26	13	11	24
Accrued Expenses [Abstract]
Compensation-related accruals	16	19
Taxes accrued	10	14
Interest accrued	21	8
Atlantic City Electric Company [Member]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	13	12	13	7
Restricted cash	4	2	3	4
Restricted cash included in other long-term assets	10	14	15	19
Total cash, cash equivalents and restricted cash	27	28	$ 31	$ 30
Accrued Expenses [Abstract]
Compensation-related accruals	14	15
Taxes accrued	3	8
Interest accrued	$ 21	$ 12