Supplemental Financial Information - Supplemental Balance Sheet Information (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	$ 587	$ 1,349	$ 898	$ 635
Restricted cash	358	247	207	253
Restricted cash included in other long-term assets	177	185	85	26
Total cash, cash equivalents and restricted cash	1,122	1,781	1,190	914
Contracts Receivable [Abstract]
Unbilled customer revenues	1,535	1,656
Investments [Abstract]
Total equity method investments		267
Equity investments without readily determinable fair values	69	72
Other Investments		2
Total Investments	464	625
Accrued Expenses [Abstract]
Compensation-related accruals	1,052	1,191
Taxes accrued	414	412
Interest accrued	337	334
Distributed Energy Company [Member]
Investments [Abstract]
Equity Method Investments		180
Other Equity Method Investments
Investments [Abstract]
Equity Method Investments	92	87
Employee benefit trusts and investments
Investments [Abstract]
Available for sale investments	262	244
Other available for sale investments
Investments [Abstract]
Available for sale investments	41	40
Exelon Generation Co L L C [Member]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	303	750	416	290
Restricted cash	146	153	138	158
Restricted cash included in other long-term assets	0	0	0	0
Total cash, cash equivalents and restricted cash	449	903	554	448
Contracts Receivable [Abstract]
Unbilled customer revenues	807	965
Investments [Abstract]
Total equity method investments		251
Equity investments without readily determinable fair values	69	72
Other Investments		2
Total Investments	235	414
Accrued Expenses [Abstract]
Compensation-related accruals	422	479
Taxes accrued	222	226
Interest accrued	65	77
Exelon Generation Co L L C [Member] | Distributed Energy Company [Member]
Investments [Abstract]
Equity Method Investments		180
Exelon Generation Co L L C [Member] | Other Equity Method Investments
Investments [Abstract]
Equity Method Investments	71	71
Exelon Generation Co L L C [Member] | Employee benefit trusts and investments
Investments [Abstract]
Available for sale investments	54	49
Exelon Generation Co L L C [Member] | Other available for sale investments
Investments [Abstract]
Available for sale investments	41	40
Commonwealth Edison Co [Member]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	90	135	76	56
Restricted cash	150	29	5	2
Restricted cash included in other long-term assets	163	166	63	0
Total cash, cash equivalents and restricted cash	403	330	144	58
Contracts Receivable [Abstract]
Unbilled customer revenues	218	223
Investments [Abstract]
Total equity method investments		6
Equity investments without readily determinable fair values	0	0
Total Investments	6	6
Accrued Expenses [Abstract]
Compensation-related accruals	171	187
Taxes accrued	83	71
Interest accrued	110	105
Commonwealth Edison Co [Member] | Distributed Energy Company [Member]
Investments [Abstract]
Equity Method Investments		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	21	130	271	63
Restricted cash	6	5	4	4
Restricted cash included in other long-term assets	0	0	0	0
Total cash, cash equivalents and restricted cash	27	135	275	67
Contracts Receivable [Abstract]
Unbilled customer revenues	146	114
Investments [Abstract]
Total equity method investments		8
Equity investments without readily determinable fair values	0	0
Total Investments	27	25
Accrued Expenses [Abstract]
Compensation-related accruals	58	49
Taxes accrued	3	28
Interest accrued	37	33
PECO Energy Co [Member] | Distributed Energy Company [Member]
Investments [Abstract]
Equity Method Investments		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	19	17
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	24	7	17	23
Restricted cash	1	6	1	24
Restricted cash included in other long-term assets	0	0	0	3
Total cash, cash equivalents and restricted cash	25	13	18	50
Contracts Receivable [Abstract]
Unbilled customer revenues	170	168
Investments [Abstract]
Total equity method investments		0
Equity investments without readily determinable fair values	0	0
Total Investments	7	5
Accrued Expenses [Abstract]
Compensation-related accruals	78	68
Taxes accrued	26	46
Interest accrued	46	39
Baltimore Gas and Electric Company [Member] | Distributed Energy Company [Member]
Investments [Abstract]
Equity Method Investments		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	7	5
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	131	124	30	170
Restricted cash	36	43	42	43
Restricted cash included in other long-term assets	14	19	23	23
Total cash, cash equivalents and restricted cash	181	186	95	236
Contracts Receivable [Abstract]
Unbilled customer revenues	194	186
Investments [Abstract]
Total equity method investments		0
Equity investments without readily determinable fair values	0	0
Total Investments	135	130
Accrued Expenses [Abstract]
Compensation-related accruals	101	99
Taxes accrued	117	74
Interest accrued	49	50
Pepco Holdings LLC [Member] | Distributed Energy Company [Member]
Investments [Abstract]
Equity Method Investments		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	135	130
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	16	5	9
Restricted cash	33	37	35	33
Restricted cash included in other long-term assets	0	0	0	0
Total cash, cash equivalents and restricted cash	63	53	40	42
Contracts Receivable [Abstract]
Unbilled customer revenues	100	97
Investments [Abstract]
Total equity method investments		0
Equity investments without readily determinable fair values	0	0
Total Investments	110	105
Accrued Expenses [Abstract]
Compensation-related accruals	28	29
Taxes accrued	90	58
Interest accrued	23	25
Potomac Electric Power Company [Member] | Distributed Energy Company [Member]
Investments [Abstract]
Equity Method Investments		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	110	105
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	13	23	2	46
Restricted cash	0	1	0	0
Restricted cash included in other long-term assets	0	0	0	0
Total cash, cash equivalents and restricted cash	13	24	2	46
Contracts Receivable [Abstract]
Unbilled customer revenues	61	59
Investments [Abstract]
Total equity method investments		0
Equity investments without readily determinable fair values	0	0
Total Investments	0	0
Accrued Expenses [Abstract]
Compensation-related accruals	19	19
Taxes accrued	14	4
Interest accrued	8	8
Delmarva Power & Light Company [Member] | Distributed Energy Company [Member]
Investments [Abstract]
Equity Method Investments		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	12	7	2	101
Restricted cash	2	4	6	9
Restricted cash included in other long-term assets	14	19	23	23
Total cash, cash equivalents and restricted cash	28	30	$ 31	$ 133
Contracts Receivable [Abstract]
Unbilled customer revenues	33	30
Investments [Abstract]
Total equity method investments		0
Equity investments without readily determinable fair values	0	0
Total Investments	0	0
Accrued Expenses [Abstract]
Compensation-related accruals	15	12
Taxes accrued	8	5
Interest accrued	12	12
Atlantic City Electric Company [Member] | Distributed Energy Company [Member]
Investments [Abstract]
Equity Method Investments		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