Supplemental Financial Information - Balance Sheet (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	$ 1,457	$ 587	$ 880	$ 1,349
Restricted cash	414	358	223	247
Restricted cash included in other long-term assets	121	177	211	185
Total cash, cash equivalents and restricted cash	1,992	1,122	1,314	1,781
Accrued Expenses [Abstract]
Compensation-related accruals	557	1,052
Taxes accrued	442	414
Interest accrued	407	337
Exelon Generation Co L L C [Member]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	821	303	537	750
Restricted cash	150	146	139	153
Restricted cash included in other long-term assets	0	0	0	0
Total cash, cash equivalents and restricted cash	971	449	676	903
Accrued Expenses [Abstract]
Compensation-related accruals	207	422
Taxes accrued	244	222
Interest accrued	84	65
Commonwealth Edison Co [Member]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	514	90	68	135
Restricted cash	211	150	17	29
Restricted cash included in other long-term assets	108	163	193	166
Total cash, cash equivalents and restricted cash	833	403	278	330
Accrued Expenses [Abstract]
Compensation-related accruals	87	171
Taxes accrued	85	83
Interest accrued	69	110
PECO Energy Co [Member]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	31	21	41	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	38	27	47	135
Accrued Expenses [Abstract]
Compensation-related accruals	39	58
Taxes accrued	1	3
Interest accrued	33	37
Baltimore Gas and Electric Company [Member]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	11	24	12	7
Restricted cash	1	1	4	6
Restricted cash included in other long-term assets	0	0	0	0
Total cash, cash equivalents and restricted cash	12	25	16	13
Accrued Expenses [Abstract]
Compensation-related accruals	44	78
Taxes accrued	43	26
Interest accrued	38	46
Pepco Holdings LLC [Member]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	49	131	33	124
Restricted cash	37	36	39	43
Restricted cash included in other long-term assets	12	14	19	19
Total cash, cash equivalents and restricted cash	98	181	91	186
Accrued Expenses [Abstract]
Compensation-related accruals	63	101
Taxes accrued	114	117
Interest accrued	78	49
Potomac Electric Power Company [Member]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	18	30	11	16
Restricted cash	33	33	35	37
Restricted cash included in other long-term assets	0	0	0	0
Total cash, cash equivalents and restricted cash	51	63	46	53
Accrued Expenses [Abstract]
Compensation-related accruals	22	28
Taxes accrued	79	90
Interest accrued	37	23
Delmarva Power and Light Company [Member]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	7	13	7	23
Restricted cash	0	0	1	1
Restricted cash included in other long-term assets	0	0	0	0
Total cash, cash equivalents and restricted cash	7	13	8	24
Accrued Expenses [Abstract]
Compensation-related accruals	14	19
Taxes accrued	25	14
Interest accrued	20	8
Atlantic City Electric Company [Member]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	8	12	6	7
Restricted cash	3	2	3	4
Restricted cash included in other long-term assets	12	14	19	19
Total cash, cash equivalents and restricted cash	23	28	$ 28	$ 30
Accrued Expenses [Abstract]
Compensation-related accruals	10	15
Taxes accrued	7	8
Interest accrued	$ 19	$ 12