Income Taxes - Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized Tax Benefits - Beginning Balance	$ 477	$ 743	$ 916
Increases based on tax positions prior to current year	34	30	28
Unrecognized Tax Benefits, Period Increase (Decrease)	26	15
Increases based on tax positions related to current year	2
Decreases based on tax positions prior to current year	3	251	196
Decrease from settlements with taxing authorities	(29)	(53)	(5)
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations		(7)
Unrecognized tax benefits - Ending Balance	507	477	743
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	408	468	490
Increases based on tax positions prior to current year	19	21	0
Unrecognized Tax Benefits, Period Increase (Decrease)	12	15
Increases based on tax positions related to current year	1
Decreases based on tax positions prior to current year	3	36	17
Decrease from settlements with taxing authorities	4	(53)	(5)
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations		(7)
Unrecognized tax benefits - Ending Balance	441	408	468
Commonwealth Edison Co [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized Tax Benefits - Beginning Balance	2	2	(12)
Increases based on tax positions prior to current year	3	0	14
Unrecognized Tax Benefits, Period Increase (Decrease)	3	0
Increases based on tax positions related to current year	0
Decreases based on tax positions prior to current year	0	0	0
Decrease from settlements with taxing authorities	(2)	0	0
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations		0
Unrecognized tax benefits - Ending Balance	6	2	2
PECO Energy Co [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized Tax Benefits - Beginning Balance	0	0	0
Increases based on tax positions prior to current year	2	0	0
Unrecognized Tax Benefits, Period Increase (Decrease)	1	0
Increases based on tax positions related to current year	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	3	0	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	0	120	120
Increases based on tax positions prior to current year	3	0	0
Unrecognized Tax Benefits, Period Increase (Decrease)	4	0
Increases based on tax positions related to current year	0
Decreases based on tax positions prior to current year	0	120	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	7	0	120
Pepco Holdings LLC [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized Tax Benefits - Beginning Balance	45	125	172
Increases based on tax positions prior to current year	0	8	14
Unrecognized Tax Benefits, Period Increase (Decrease)	3	0
Increases based on tax positions related to current year	0
Decreases based on tax positions prior to current year	0	88	61
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	48	45	125
Potomac Electric Power Company [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized Tax Benefits - Beginning Balance	0	59	80
Increases based on tax positions prior to current year	0	7	0
Unrecognized Tax Benefits, Period Increase (Decrease)	2	0
Increases based on tax positions related to current year	0
Decreases based on tax positions prior to current year	0	66	21
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	2	0	59
Delmarva Power & Light Company [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized Tax Benefits - Beginning Balance	0	21	37
Increases based on tax positions prior to current year	0	1	0
Unrecognized Tax Benefits, Period Increase (Decrease)	1	0
Increases based on tax positions related to current year	0
Decreases based on tax positions prior to current year	0	22	16
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	0	21
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	22
Increases based on tax positions prior to current year	0	0	14
Unrecognized Tax Benefits, Period Increase (Decrease)	0	0
Increases based on tax positions related to current year	0
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	14	$ 14	14
Internal Revenue Service (IRS) [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit			146
Internal Revenue Service (IRS) [Member] | Exelon Generation Co L L C [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit			19
Internal Revenue Service (IRS) [Member] | Pepco Holdings LLC [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit			59
Internal Revenue Service (IRS) [Member] | Potomac Electric Power Company [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit			21
Internal Revenue Service (IRS) [Member] | Delmarva Power & Light Company [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit			16
Internal Revenue Service (IRS) [Member] | Atlantic City Electric Company [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit			$ 22
State and Local Jurisdiction [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit	425
State and Local Jurisdiction [Member] | Exelon Generation Co L L C [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit	411
State and Local Jurisdiction [Member] | Commonwealth Edison Co [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit	0
State and Local Jurisdiction [Member] | PECO Energy Co [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit	0
State and Local Jurisdiction [Member] | Baltimore Gas and Electric Company [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit	0
State and Local Jurisdiction [Member] | Pepco Holdings LLC [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit	0
State and Local Jurisdiction [Member] | Potomac Electric Power Company [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit	0
State and Local Jurisdiction [Member] | Delmarva Power & Light Company [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit	0
State and Local Jurisdiction [Member] | Atlantic City Electric Company [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit	$ 14