Income Taxes - Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Unrecognized Tax Benefits, Increase Resulting from Acquisition			$ 22
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits - beginning balance	$ 743	$ 916	1,078
Increases based on tax positions related to current year			108
Increases based on tax positions prior to current year	15		(332)
Decreases based on tax positions prior to current year	30	28	88
Unrecognized Tax Benefits, Period Increase (Decrease)	(251)	(196)	(21)
Decrease from settlements with taxing authorities	(53)	(5)	(27)
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations	(7)
Unrecognized tax benefits - ending balance	477	743	916
Exelon Generation Co L L C [Member]
Income Taxes [Line Items]
Unrecognized Tax Benefits, Increase Resulting from Acquisition			5
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits - beginning balance	468	490	534
Increases based on tax positions related to current year			10
Increases based on tax positions prior to current year	15		(12)
Decreases based on tax positions prior to current year	21	0	0
Unrecognized Tax Benefits, Period Increase (Decrease)	(36)	(17)	(20)
Decrease from settlements with taxing authorities	(53)	(5)	(27)
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations	(7)
Unrecognized tax benefits - ending balance	408	468	490
Commonwealth Edison Co [Member]
Income Taxes [Line Items]
Unrecognized Tax Benefits, Increase Resulting from Acquisition			0
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits - beginning balance	2	(12)	142
Increases based on tax positions related to current year			0
Increases based on tax positions prior to current year	0		(154)
Decreases based on tax positions prior to current year	0	14	0
Unrecognized Tax Benefits, Period Increase (Decrease)	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	2	2	(12)
PECO Energy Co [Member]
Income Taxes [Line Items]
Unrecognized Tax Benefits, Increase Resulting from Acquisition			0
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 related to current year			0
Increases based on tax positions prior to current year	0		0
Decreases based on tax positions prior to current year	0	0	0
Unrecognized Tax Benefits, Period Increase (Decrease)	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	0	0	0
Baltimore Gas and Electric Company [Member]
Income Taxes [Line Items]
Unrecognized Tax Benefits, Increase Resulting from Acquisition			0
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits - beginning balance	120	120	120
Increases based on tax positions related to current year			0
Increases based on tax positions prior to current year	0		0
Decreases based on tax positions prior to current year	0	0	0
Unrecognized Tax Benefits, Period Increase (Decrease)	(120)	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	0	120	120
Potomac Electric Power Company [Member]
Income Taxes [Line Items]
Unrecognized Tax Benefits, Increase Resulting from Acquisition			0
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits - beginning balance	59	80	8
Increases based on tax positions related to current year			21
Increases based on tax positions prior to current year	0		0
Decreases based on tax positions prior to current year	7	0	51
Unrecognized Tax Benefits, Period Increase (Decrease)	(66)	(21)	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	0	59	80
Delmarva Power & Light Company [Member]
Income Taxes [Line Items]
Unrecognized Tax Benefits, Increase Resulting from Acquisition			0
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits - beginning balance	21	37	3
Increases based on tax positions related to current year			16
Increases based on tax positions prior to current year	0		0
Decreases based on tax positions prior to current year	1	0	18
Unrecognized Tax Benefits, Period Increase (Decrease)	(22)	(16)	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	0	21	37
Atlantic City Electric Company [Member]
Income Taxes [Line Items]
Unrecognized Tax Benefits, Increase Resulting from Acquisition			0
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits - beginning balance	14	22	0
Increases based on tax positions related to current year			22
Increases based on tax positions prior to current year	0		0
Decreases based on tax positions prior to current year	0	14	0
Unrecognized Tax Benefits, Period Increase (Decrease)	0	(22)	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	14	14	22
Pepco Holdings LLC [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits - beginning balance	125	172
Increases based on tax positions prior to current year	0
Decreases based on tax positions prior to current year	8	14
Unrecognized Tax Benefits, Period Increase (Decrease)	(88)	(61)
Decrease from settlements with taxing authorities	0	0
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations	0
Unrecognized tax benefits - ending balance	$ 45	125	172
Pepco Holdings LLC [Member] | Predecessor [Member]
Income Taxes [Line Items]
Unrecognized Tax Benefits, Increase Resulting from Acquisition			(5)
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits - beginning balance		$ 172	22
Increases based on tax positions related to current year			59
Increases based on tax positions prior to current year			0
Decreases based on tax positions prior to current year			96
Unrecognized Tax Benefits, Period Increase (Decrease)			0
Decrease from settlements with taxing authorities			0
Unrecognized tax benefits - ending balance			$ 172