Income Taxes - Reconciliation of Unrecognized Tax Benefits (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits - beginning balance		$ 2,175	$ 1,024	$ 807
Merger balance transfer				195
Increases based on tax positions related to current year		15	19	34
Changes to tax positions that only affect timing		(255)	649	(88)
Increases based on tax positions prior to current year		18	493	91
Decreases based on tax positions prior to current year		(1)	(6)	(6)
Decrease from settlements with taxing authorities		(35)		(2)
Decreases from expiration of statute of limitations		(88)	(4)	(7)
Unrecognized tax benefits - ending balance	1,829	1,829	2,175	1,024
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		1,415	876	683
Merger balance transfer				183
Increases based on tax positions related to current year		15	19	3
Changes to tax positions that only affect timing		33	36	(69)
Increases based on tax positions prior to current year		18	493	91
Decreases based on tax positions prior to current year	(661)	(2)	(5)	(6)
Decrease from settlements with taxing authorities		(34)		(2)
Decreases from expiration of statute of limitations		(88)	(4)	(7)
Unrecognized tax benefits - ending balance	1,357	1,357	1,415	876
Commonwealth Edison Co [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits - beginning balance		324	67	70
Merger balance transfer				0
Increases based on tax positions related to current year		0	0	0
Changes to tax positions that only affect timing		(175)	257	(3)
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
Decreases from expiration of statute of limitations		0	0	0
Unrecognized tax benefits - ending balance	149	149	324	67
PECO Energy Co [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits - beginning balance		44	44	48
Merger balance transfer				0
Increases based on tax positions related to current year		0	0	0
Changes to tax positions that only affect timing		0	0	(4)
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
Decreases from expiration of statute of limitations		0	0	0
Unrecognized tax benefits - ending balance	44	44	44	44
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	0	11
Merger balance transfer				0
Increases based on tax positions related to current year		0	0	0
Changes to tax positions that only affect timing		0	0	(11)
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
Decreases from expiration of statute of limitations		0	0	0
Unrecognized tax benefits - ending balance	$ 0	$ 0	$ 0	$ 0