Supplemental Financial Information - Supplemental Balance Sheet Information (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	$ 663	$ 587	$ 1,349	$ 898
Restricted cash	438	358	247	207
Restricted cash included in other long-term assets	53	177	185	85
Total cash, cash equivalents and restricted cash	1,166	1,122	1,781	1,190
Contracts Receivable [Abstract]
Unbilled customer revenues	998	1,535
Investments [Abstract]
Equity investments without readily determinable fair values	73	69
Total Investments	440	464
Accrued Expenses [Abstract]
Compensation-related accruals	1,069	1,052
Taxes accrued	527	414
Interest accrued	331	337
Held for Sale
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Total cash, cash equivalents and restricted cash	12
Other Equity Method Investments
Investments [Abstract]
Equity Method Investments	81	92
Employee benefit trusts and investments
Investments [Abstract]
Available for sale investments	283	262
Other available for sale investments
Investments [Abstract]
Available for sale investments	3	41
Exelon Generation Co L L C [Member]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	226	303	750	416
Restricted cash	89	146	153	138
Restricted cash included in other long-term assets	0	0	0	0
Total cash, cash equivalents and restricted cash	327	449	903	554
Contracts Receivable [Abstract]
Unbilled customer revenues	258	807
Investments [Abstract]
Equity investments without readily determinable fair values	55	69
Total Investments	184	235
Accrued Expenses [Abstract]
Compensation-related accruals	426	422
Taxes accrued	229	222
Interest accrued	44	65
Exelon Generation Co L L C [Member] | Held for Sale
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Total cash, cash equivalents and restricted cash	12
Exelon Generation Co L L C [Member] | Other Equity Method Investments
Investments [Abstract]
Equity Method Investments	65	71
Exelon Generation Co L L C [Member] | Employee benefit trusts and investments
Investments [Abstract]
Available for sale investments	61	54
Exelon Generation Co L L C [Member] | Other available for sale investments
Investments [Abstract]
Available for sale investments	3	41
Commonwealth Edison Co [Member]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	83	90	135	76
Restricted cash	279	150	29	5
Restricted cash included in other long-term assets	43	163	166	63
Total cash, cash equivalents and restricted cash	405	403	330	144
Contracts Receivable [Abstract]
Unbilled customer revenues	218	218
Investments [Abstract]
Equity investments without readily determinable fair values	0	0
Total Investments	6	6
Accrued Expenses [Abstract]
Compensation-related accruals	170	171
Taxes accrued	94	83
Interest accrued	109	110
Commonwealth Edison Co [Member] | Held for Sale
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Total cash, cash equivalents and restricted cash	0
Commonwealth Edison Co [Member] | Other Equity Method Investments
Investments [Abstract]
Equity Method Investments	6	6
Commonwealth Edison Co [Member] | Employee benefit trusts and investments
Investments [Abstract]
Available for sale investments	0	0
Commonwealth Edison Co [Member] | Other available for sale investments
Investments [Abstract]
Available for sale investments	0	0
PECO Energy Co [Member]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	19	21	130	271
Restricted cash	7	6	5	4
Restricted cash included in other long-term assets	0	0	0	0
Total cash, cash equivalents and restricted cash	26	27	135	275
Contracts Receivable [Abstract]
Unbilled customer revenues	147	146
Investments [Abstract]
Equity investments without readily determinable fair values	0	0
Total Investments	30	27
Accrued Expenses [Abstract]
Compensation-related accruals	73	58
Taxes accrued	16	3
Interest accrued	37	37
PECO Energy Co [Member] | Held for Sale
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Total cash, cash equivalents and restricted cash	0
PECO Energy Co [Member] | Other Equity Method Investments
Investments [Abstract]
Equity Method Investments	8	8
PECO Energy Co [Member] | Employee benefit trusts and investments
Investments [Abstract]
Available for sale investments	22	19
PECO Energy Co [Member] | Other available for sale investments
Investments [Abstract]
Available for sale investments	0	0
Baltimore Gas and Electric Company [Member]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	144	24	7	17
Restricted cash	1	1	6	1
Restricted cash included in other long-term assets	0	0	0	0
Total cash, cash equivalents and restricted cash	145	25	13	18
Contracts Receivable [Abstract]
Unbilled customer revenues	197	170
Investments [Abstract]
Equity investments without readily determinable fair values	0	0
Total Investments	10	7
Accrued Expenses [Abstract]
Compensation-related accruals	84	78
Taxes accrued	73	26
Interest accrued	46	46
Baltimore Gas and Electric Company [Member] | Held for Sale
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Total cash, cash equivalents and restricted cash	0
Baltimore Gas and Electric Company [Member] | Other Equity Method Investments
Investments [Abstract]
Equity Method Investments	0	0
Baltimore Gas and Electric Company [Member] | Employee benefit trusts and investments
Investments [Abstract]
Available for sale investments	10	7
Baltimore Gas and Electric Company [Member] | Other available for sale investments
Investments [Abstract]
Available for sale investments	0	0
Pepco Holdings LLC [Member]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	111	131	124	30
Restricted cash	39	36	43	42
Restricted cash included in other long-term assets	10	14	19	23
Total cash, cash equivalents and restricted cash	160	181	186	95
Contracts Receivable [Abstract]
Unbilled customer revenues	178	194
Investments [Abstract]
Equity investments without readily determinable fair values	0	0
Total Investments	140	135
Accrued Expenses [Abstract]
Compensation-related accruals	109	101
Taxes accrued	117	117
Interest accrued	51	49
Pepco Holdings LLC [Member] | Held for Sale
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Total cash, cash equivalents and restricted cash	0
Pepco Holdings LLC [Member] | Other Equity Method Investments
Investments [Abstract]
Equity Method Investments	0	0
Pepco Holdings LLC [Member] | Employee benefit trusts and investments
Investments [Abstract]
Available for sale investments	140	135
Pepco Holdings LLC [Member] | Other available for sale investments
Investments [Abstract]
Available for sale investments	0	0
Potomac Electric Power Company [Member]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	30	30	16	5
Restricted cash	35	33	37	35
Restricted cash included in other long-term assets	0	0	0	0
Total cash, cash equivalents and restricted cash	65	63	53	40
Contracts Receivable [Abstract]
Unbilled customer revenues	87	100
Investments [Abstract]
Equity investments without readily determinable fair values	0	0
Total Investments	115	110
Accrued Expenses [Abstract]
Compensation-related accruals	36	28
Taxes accrued	90	90
Interest accrued	26	23
Potomac Electric Power Company [Member] | Held for Sale
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Total cash, cash equivalents and restricted cash	0
Potomac Electric Power Company [Member] | Other Equity Method Investments
Investments [Abstract]
Equity Method Investments	0	0
Potomac Electric Power Company [Member] | Employee benefit trusts and investments
Investments [Abstract]
Available for sale investments	115	110
Potomac Electric Power Company [Member] | Other available for sale investments
Investments [Abstract]
Available for sale investments	0	0
Delmarva Power & Light Company [Member]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	15	13	23	2
Restricted cash	0	0	1	0
Restricted cash included in other long-term assets	0	0	0	0
Total cash, cash equivalents and restricted cash	15	13	24	2
Contracts Receivable [Abstract]
Unbilled customer revenues	62	61
Investments [Abstract]
Equity investments without readily determinable fair values	0	0
Total Investments	0	0
Accrued Expenses [Abstract]
Compensation-related accruals	18	19
Taxes accrued	18	14
Interest accrued	7	8
Delmarva Power & Light Company [Member] | Held for Sale
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Total cash, cash equivalents and restricted cash	0
Delmarva Power & Light Company [Member] | Other Equity Method Investments
Investments [Abstract]
Equity Method Investments	0	0
Delmarva Power & Light Company [Member] | Employee benefit trusts and investments
Investments [Abstract]
Available for sale investments	0	0
Delmarva Power & Light Company [Member] | Other available for sale investments
Investments [Abstract]
Available for sale investments	0	0
Atlantic City Electric Company [Member]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	17	12	7	2
Restricted cash	3	2	4	6
Restricted cash included in other long-term assets	10	14	19	23
Total cash, cash equivalents and restricted cash	30	28	$ 30	$ 31
Contracts Receivable [Abstract]
Unbilled customer revenues	29	33
Investments [Abstract]
Equity investments without readily determinable fair values	0	0
Total Investments	0	0
Accrued Expenses [Abstract]
Compensation-related accruals	17	15
Taxes accrued	12	8
Interest accrued	12	12
Atlantic City Electric Company [Member] | Held for Sale
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Total cash, cash equivalents and restricted cash	0
Atlantic City Electric Company [Member] | Other Equity Method Investments
Investments [Abstract]
Equity Method Investments	0	0
Atlantic City Electric Company [Member] | Employee benefit trusts and investments
Investments [Abstract]
Available for sale investments	0	0
Atlantic City Electric Company [Member] | Other available for sale investments
Investments [Abstract]
Available for sale investments	$ 0	$ 0