Derivative Financial Instruments Derivative Financial Instruments - Summary of Derivative Fair Value Balances (Details) - USD ($) $ in Millions		Sep. 30, 2016	Mar. 23, 2016	Dec. 31, 2015
Derivative [Line Items]
Derivative Asset, Current		$ 754		$ 1,365
Derivative Asset, Noncurrent		669		758
Derivative Liability, Current		(222)		(205)
Derivative Liability, Noncurrent		(422)		(374)
Derivative [Member]
Derivative [Line Items]
Derivative Asset, Current		745		1,350
Derivative Asset, Noncurrent		624		719
Total mark-to-market derivative net assets (liabilities)		744		1,506
Total mark-to-market derivative assets		1,369		2,069
Derivative Liability, Current		(213)		(197)
Derivative Liability, Noncurrent		(412)		(366)
Total mark-to-market derivative liabilities		(625)		(563)
Exelon Generation Co L L C [Member]
Derivative [Line Items]
Derivative Asset, Current		754		1,365
Derivative Asset, Noncurrent		630		733
Derivative Liability, Current		(203)		(182)
Derivative Liability, Noncurrent		(197)		(150)
Exelon Generation Co L L C [Member] | Economic Hedging Instrument [Member]
Derivative [Line Items]
Derivative Asset, Current		3,482		5,236
Derivative Asset, Noncurrent		2,139		1,860
Total mark-to-market derivative net assets (liabilities)		507		516
Total mark-to-market derivative assets		5,621		7,096
Derivative Liability, Current		(3,229)		(4,907)
Derivative Liability, Noncurrent		(1,885)		(1,673)
Total mark-to-market derivative liabilities		(5,114)		(6,580)
Exelon Generation Co L L C [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Current assets collateral offset		135		352
Noncurrent assets collateral offset		84		180
Current liabilities collateral offset		156		480
Noncurrent liabilities collateral offset		110		222
Total cash collateral received net of cash collateral posted		485		(1,234)
Exelon Generation Co L L C [Member] | Derivative [Member]
Derivative [Line Items]
Derivative Asset, Current	[1]	745		1,350
Derivative Asset, Noncurrent	[1]	624		719
Total mark-to-market derivative net assets (liabilities)	[1]	988		1,753
Total mark-to-market derivative assets	[1]	1,369		2,069
Derivative Liability, Current	[1]	(194)		(174)
Derivative Liability, Noncurrent	[1]	(187)		(142)
Total mark-to-market derivative liabilities	[1]	(381)		(316)
Exelon Generation Co L L C [Member] | Proprietary Trading [Member]
Derivative [Line Items]
Derivative Asset, Current		67		108
Derivative Asset, Noncurrent		31		22
Total mark-to-market derivative net assets (liabilities)		(4)		3
Total mark-to-market derivative assets		98		130
Derivative Liability, Current		(61)		(94)
Derivative Liability, Noncurrent		(41)		(33)
Total mark-to-market derivative liabilities		(102)		(127)
Exelon Generation Co L L C [Member] | Collateral And Netting [Member]
Derivative [Line Items]
Derivative Asset, Current		(2,804)		(3,994)
Derivative Asset, Noncurrent		(1,546)		(1,163)
Total mark-to-market derivative net assets (liabilities)	[2]	485		1,234
Total mark-to-market derivative assets	[2]	(4,350)		(5,157)
Derivative Liability, Current		3,096		4,827
Derivative Liability, Noncurrent		1,739		1,564
Total mark-to-market derivative liabilities	[2]	4,835		6,391
Commonwealth Edison Co [Member]
Derivative [Line Items]
Derivative Liability, Current		(19)		(23)
Derivative Liability, Noncurrent		(225)		(224)
Commonwealth Edison Co [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Asset, Current		0		0
Derivative Asset, Noncurrent		0		0
Total mark-to-market derivative net assets (liabilities)	[3]	(244)		(247)
Total mark-to-market derivative assets	[3]	0		0
Derivative Liability, Current		19		(23)
Derivative Liability, Noncurrent		225		(224)
Total mark-to-market derivative liabilities	[3]	(244)		(247)
Delmarva Power and Light Company [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Asset, Current		1		0
Derivative Asset, Noncurrent		0		0
Total mark-to-market derivative net assets (liabilities)		1		(2)
Total mark-to-market derivative assets		1		0
Derivative Liability, Current		0		2
Derivative Liability, Noncurrent		0		0
Total mark-to-market derivative liabilities		0		2
Delmarva Power and Light Company [Member] | Derivative [Member]
Derivative [Line Items]
Derivative Asset, Current	[4]	0		0
Derivative Asset, Noncurrent	[4]	0		0
Total mark-to-market derivative net assets (liabilities)		0		0
Total mark-to-market derivative assets	[4]	0		0
Derivative Liability, Current	[4]	0		0
Derivative Liability, Noncurrent	[4]	0		0
Total mark-to-market derivative liabilities	[4]	0		0
Delmarva Power and Light Company [Member] | Collateral And Netting [Member]
Derivative [Line Items]
Derivative Asset, Current		(1)		0
Derivative Asset, Noncurrent		0		0
Total mark-to-market derivative net assets (liabilities)		(1)		2
Total mark-to-market derivative assets		(1)		0
Derivative Liability, Current		0		(2)
Derivative Liability, Noncurrent		0		0
Total mark-to-market derivative liabilities		0		(2)
Pepco Holdings LLC [Member]
Derivative [Line Items]
Total mark-to-market derivative net assets (liabilities)			$ 0
Pepco Holdings LLC [Member] | Successor [Member]
Derivative [Line Items]
Derivative Asset, Current		0
Derivative Asset, Noncurrent		0
Total mark-to-market derivative net assets (liabilities)		0
Total mark-to-market derivative assets		0
Derivative Liability, Current		0
Derivative Liability, Noncurrent		0
Total mark-to-market derivative liabilities		$ 0
Pepco Holdings LLC [Member] | Predecessor [Member]
Derivative [Line Items]
Derivative Asset, Current				18
Derivative Asset, Noncurrent				0
Total mark-to-market derivative net assets (liabilities)				18
Total mark-to-market derivative assets				18
Derivative Liability, Current				0
Derivative Liability, Noncurrent				0
Total mark-to-market derivative liabilities				0
Pepco Holdings LLC [Member] | Derivative [Member] | Corporate, Non-Segment [Member]
Derivative [Line Items]
Derivative Asset, Current				18
Derivative Asset, Noncurrent				0
Total mark-to-market derivative net assets (liabilities)				18
Total mark-to-market derivative assets				18
Derivative Liability, Current				0
Derivative Liability, Noncurrent				0
Total mark-to-market derivative liabilities				$ 0

[1] .

[2] .

[3] .

[4] Current and noncurrent assets are shown net of collateral of $352 million and $180 million, respectively, and current and noncurrent liabilities are shown net of collateral of $480 million and $222 million, respectively. The total cash collateral posted, net of cash collateral received and offset against mark-to-market assets and liabilities was $1,234 million at December 31, 2015.