Income Taxes - Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized Tax Benefits - Beginning Balance	$ 507	$ 477	$ 743
Unrecognized Tax Benefits, Period Increase (Decrease)	6	26	15
Increases based on tax positions related to current year	3	2
Increases based on tax positions prior to current year	26	34	30
Decreases based on tax positions prior to current year	348	3	251
Decrease from settlements with taxing authorities	(69)	(29)	(53)
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations			(7)
Unrecognized tax benefits - Ending Balance	125	507	477
Decrease in Unrecognized Tax Benefits is Reasonably Possible	411
Tax Adjustments, Settlements, and Unusual Provisions	76
Exelon Generation Co L L C [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized Tax Benefits - Beginning Balance	441	408	468
Unrecognized Tax Benefits, Period Increase (Decrease)	0	12	15
Increases based on tax positions related to current year	1	1
Increases based on tax positions prior to current year	23	19	21
Decreases based on tax positions prior to current year	346	3	36
Decrease from settlements with taxing authorities	(69)	4	(53)
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations			(7)
Unrecognized tax benefits - Ending Balance	50	441	408
Decrease in Unrecognized Tax Benefits is Reasonably Possible	411
Tax Adjustments, Settlements, and Unusual Provisions	73
Commonwealth Edison Co [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized Tax Benefits - Beginning Balance	6	2	2
Unrecognized Tax Benefits, Period Increase (Decrease)	2	3	0
Increases based on tax positions related to current year	0	0
Increases based on tax positions prior to current year	1	3	0
Decreases based on tax positions prior to current year	0	0	0
Decrease from settlements with taxing authorities	0	(2)	0
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations			0
Unrecognized tax benefits - Ending Balance	9	6	2
PECO Energy Co [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized Tax Benefits - Beginning Balance	3	0	0
Unrecognized Tax Benefits, Period Increase (Decrease)	3	1	0
Increases based on tax positions related to current year	0	0
Increases based on tax positions prior to current year	0	2	0
Decreases based on tax positions prior to current year	0	0	0
Decrease from settlements with taxing authorities	0	0	0
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations			0
Unrecognized tax benefits - Ending Balance	6	3	0
Baltimore Gas and Electric Company [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized Tax Benefits - Beginning Balance	7	0	120
Unrecognized Tax Benefits, Period Increase (Decrease)	3	4	0
Increases based on tax positions related to current year	0	0
Increases based on tax positions prior to current year	0	3	0
Decreases based on tax positions prior to current year	0	0	120
Decrease from settlements with taxing authorities	0	0	0
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations			0
Unrecognized tax benefits - Ending Balance	10	7	0
Pepco Holdings LLC [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized Tax Benefits - Beginning Balance	48	45	125
Unrecognized Tax Benefits, Period Increase (Decrease)	3	3	0
Increases based on tax positions related to current year	0	0
Increases based on tax positions prior to current year	1	0	8
Decreases based on tax positions prior to current year	0	0	88
Decrease from settlements with taxing authorities	0	0	0
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations			0
Unrecognized tax benefits - Ending Balance	52	48	45
Potomac Electric Power Company [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized Tax Benefits - Beginning Balance	2	0	59
Unrecognized Tax Benefits, Period Increase (Decrease)	1	2	0
Increases based on tax positions related to current year	0	0
Increases based on tax positions prior to current year	0	0	7
Decreases based on tax positions prior to current year	0	0	66
Decrease from settlements with taxing authorities	0	0	0
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations			0
Unrecognized tax benefits - Ending Balance	3	2	0
Delmarva Power & Light Company [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized Tax Benefits - Beginning Balance	1	0	21
Unrecognized Tax Benefits, Period Increase (Decrease)	0	1	0
Increases based on tax positions related to current year	0	0
Increases based on tax positions prior to current year	0	0	1
Decreases based on tax positions prior to current year	0	0	22
Decrease from settlements with taxing authorities	0	0	0
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations			0
Unrecognized tax benefits - Ending Balance	1	1	0
Atlantic City Electric Company [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized Tax Benefits - Beginning Balance	14	14	14
Unrecognized Tax Benefits, Period Increase (Decrease)	1	0	0
Increases based on tax positions related to current year	0	0
Increases based on tax positions prior to current year	0	0	0
Decreases based on tax positions prior to current year	0	0	0
Decrease from settlements with taxing authorities	0	0	0
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations			0
Unrecognized tax benefits - Ending Balance	15	$ 14	$ 14
State and Local Jurisdiction [Member] | Pepco Holdings LLC [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Decrease in Unrecognized Tax Benefits is Reasonably Possible	$ 14