Income Taxes - Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits - beginning balance	$ 916		$ 1,078	$ 1,829
Unrecognized Tax Benefits, Increase Resulting from Acquisition			22
Increases based on tax positions related to current year	0		108	108
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions	0		0	0
Increases based on tax positions prior to current year	0		(332)	(705)
Decreases based on tax positions prior to current year	28		88	79
Unrecognized Tax Benefits, Period Increase (Decrease)	(196)	[1]	(21)	(116)
Decrease from settlements with taxing authorities	(5)		(27)	(31)
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations	0		0	(86)
Unrecognized tax benefits - ending balance	743		916	1,078
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	490		534	1,357
Unrecognized Tax Benefits, Increase Resulting from Acquisition			5
Increases based on tax positions related to current year	0		10	0
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions	0		0	0
Increases based on tax positions prior to current year	0		(12)	(659)
Decreases based on tax positions prior to current year	0		0	65
Unrecognized Tax Benefits, Period Increase (Decrease)	(17)	[1]	(20)	(112)
Decrease from settlements with taxing authorities	(5)		(27)	(31)
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations	0		0	(86)
Unrecognized tax benefits - ending balance	468		490	534
Commonwealth Edison Co [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits - beginning balance	(12)		142	149
Unrecognized Tax Benefits, Increase Resulting from Acquisition			0
Increases based on tax positions related to current year	0		0	0
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions	0		0	0
Increases based on tax positions prior to current year	0		(154)	(7)
Decreases based on tax positions prior to current year	14		0	0
Unrecognized Tax Benefits, Period Increase (Decrease)	0	[1]	0	0
Decrease from settlements with taxing authorities	0		0	0
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations	0		0	0
Unrecognized tax benefits - ending balance	2		(12)	142
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	44
Unrecognized Tax Benefits, Increase Resulting from Acquisition			0
Increases based on tax positions related to current year	0		0	0
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions	0		0	0
Increases based on tax positions prior to current year	0		0	(44)
Decreases based on tax positions prior to current year	0		0	0
Unrecognized Tax Benefits, Period Increase (Decrease)	0	[1]	0	0
Decrease from settlements with taxing authorities	0		0	0
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations	0		0	0
Unrecognized tax benefits - ending balance	0		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	120		120	0
Unrecognized Tax Benefits, Increase Resulting from Acquisition			0
Increases based on tax positions related to current year	0		0	106
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions	0		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	14
Unrecognized Tax Benefits, Period Increase (Decrease)	0	[1]	0	0
Decrease from settlements with taxing authorities	0		0	0
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations	0		0	0
Unrecognized tax benefits - ending balance	120		120	120
Potomac Electric Power Company [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits - beginning balance	80		8	0
Unrecognized Tax Benefits, Increase Resulting from Acquisition			0
Increases based on tax positions related to current year	0		21	0
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions	0		0	0
Increases based on tax positions prior to current year	0		0	0
Decreases based on tax positions prior to current year	0		51	8
Unrecognized Tax Benefits, Period Increase (Decrease)	(21)	[1]	0	0
Decrease from settlements with taxing authorities	0		0	0
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations	0		0	0
Unrecognized tax benefits - ending balance	59		80	8
Delmarva Power and Light Company [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits - beginning balance	37		3	0
Unrecognized Tax Benefits, Increase Resulting from Acquisition			0
Increases based on tax positions related to current year	0		16	0
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions	0		0	0
Increases based on tax positions prior to current year	0		0	0
Decreases based on tax positions prior to current year	0		18	3
Unrecognized Tax Benefits, Period Increase (Decrease)	(16)	[1]	0	0
Decrease from settlements with taxing authorities	0		0	0
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations	0		0	0
Unrecognized tax benefits - ending balance	21		37	3
Atlantic City Electric Company [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits - beginning balance	22		0	0
Unrecognized Tax Benefits, Increase Resulting from Acquisition			0
Increases based on tax positions related to current year	0		22	0
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions	0		0	0
Increases based on tax positions prior to current year	0		0	0
Decreases based on tax positions prior to current year	14		0	0
Unrecognized Tax Benefits, Period Increase (Decrease)	(22)	[1]	0	0
Decrease from settlements with taxing authorities	0		0	0
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations	0		0	0
Unrecognized tax benefits - ending balance	14		22	0
Pepco Holdings LLC [Member] | Successor [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits - beginning balance	172		22
Unrecognized Tax Benefits, Increase Resulting from Acquisition			(5)
Increases based on tax positions related to current year	0		59
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions	0		0
Increases based on tax positions prior to current year	0		0
Decreases based on tax positions prior to current year	14		96
Unrecognized Tax Benefits, Period Increase (Decrease)	(61)	[1]	0
Decrease from settlements with taxing authorities	0		0
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations	0		0
Unrecognized tax benefits - ending balance	$ 125		172	22
Pepco Holdings LLC [Member] | Predecessor [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits - beginning balance			$ 22	702
Increases based on tax positions related to current year				0
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions				0
Increases based on tax positions prior to current year				(688)
Decreases based on tax positions prior to current year				11
Unrecognized Tax Benefits, Period Increase (Decrease)				0
Decrease from settlements with taxing authorities				0
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations				(3)
Unrecognized tax benefits - ending balance				$ 22

[1] Predecessor Exelon Generation ComEd PECO BGE PHI Pepco DPL ACEUnrecognized tax benefits at January 1, 2015$1,829 $1,357 $149 $44 $— $702 $— $— $—Increases based on tax positions related to 2015108 — — — 106 — — — —Decreases based on tax positions related to 2015— — — — — — — — —Change to positions that only affect timing(705) (659) (7) (44) — (688) — — —Increases based on tax positions prior to 201579 65 — — 14 11 8 3 —Decreases based on tax positions prior to 2015(116) (112) — — — — — — —Decreases from settlements with taxing authorities(31) (31) — — — — — — —Decreases from expiration of statute of limitations(86) (86) — — — (3) — — —Unrecognized tax benefits at December 31, 2015$1,078 $534 $142 $— $120$22$8$3$—