Derivative Financial Instruments Derivative Financial Instruments - Summary of Derivative Fair Value Balances (Details) - USD ($) $ in Millions		Mar. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Derivative Asset, Current		$ 1,185	$ 1,365
Derivative Asset, Noncurrent		841	758
Derivative Liability, Current		(203)	(205)
Mark-to-market derivative liabilities		(408)	(374)
Derivative [Member]
Derivative [Line Items]
Derivative Asset, Current		1,172	1,350
Derivative Asset, Noncurrent		788	719
Total mark-to-market derivative net assets (liabilities)		1,375	1,506
Total mark-to-market derivative assets		1,960	2,069
Derivative Liability, Current		(195)	(197)
Mark-to-market derivative liabilities		(390)	(366)
Total mark-to-market derivative liabilities		(585)	(563)
Exelon Generation Co L L C [Member]
Derivative [Line Items]
Derivative Asset, Current		1,185	1,365
Derivative Asset, Noncurrent		799	733
Derivative Liability, Current		(177)	(182)
Mark-to-market derivative liabilities		(166)	(150)
Exelon Generation Co L L C [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Total mark-to-market derivative net assets (liabilities)		602	516
Current assets collateral offset		303	352
Noncurrent assets collateral offset		146	180
Current liabilities collateral offset		410	480
Noncurrent liabilities collateral offset		180	222
Total cash collateral received net of cash collateral posted		1,039	(1,234)
Total mark-to-market derivative assets		8,228	7,096
Total mark-to-market derivative liabilities		(7,626)	(6,580)
Exelon Generation Co L L C [Member] | Designated as Hedging Instrument [Member] | Other Current Assets [Member]
Derivative [Line Items]
Derivative Asset, Current		5,851	5,236
Exelon Generation Co L L C [Member] | Designated as Hedging Instrument [Member] | Other Noncurrent Assets [Member]
Derivative [Line Items]
Derivative Asset, Noncurrent		2,377	1,860
Exelon Generation Co L L C [Member] | Designated as Hedging Instrument [Member] | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative Liability, Current		(5,570)	(4,907)
Exelon Generation Co L L C [Member] | Designated as Hedging Instrument [Member] | Other Noncurrent Liabilities [Member]
Derivative [Line Items]
Mark-to-market derivative liabilities		(2,056)	(1,673)
Exelon Generation Co L L C [Member] | Derivative [Member]
Derivative [Line Items]
Total mark-to-market derivative net assets (liabilities)	[1]	1,640	1,753
Total mark-to-market derivative assets	[1]	1,960	2,069
Total mark-to-market derivative liabilities	[1]	(320)	(316)
Exelon Generation Co L L C [Member] | Derivative [Member] | Other Current Assets [Member]
Derivative [Line Items]
Derivative Asset, Current	[1]	1,172	1,350
Exelon Generation Co L L C [Member] | Derivative [Member] | Other Noncurrent Assets [Member]
Derivative [Line Items]
Derivative Asset, Noncurrent	[1]	788	719
Exelon Generation Co L L C [Member] | Derivative [Member] | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative Liability, Current	[1]	(169)	(174)
Exelon Generation Co L L C [Member] | Derivative [Member] | Other Noncurrent Liabilities [Member]
Derivative [Line Items]
Mark-to-market derivative liabilities	[1]	(151)	(142)
Exelon Generation Co L L C [Member] | Proprietary Trading [Member]
Derivative [Line Items]
Total mark-to-market derivative net assets (liabilities)		(1)	3
Total mark-to-market derivative assets		143	130
Total mark-to-market derivative liabilities		(144)	(127)
Exelon Generation Co L L C [Member] | Proprietary Trading [Member] | Other Current Assets [Member]
Derivative [Line Items]
Derivative Asset, Current		113	108
Exelon Generation Co L L C [Member] | Proprietary Trading [Member] | Other Noncurrent Assets [Member]
Derivative [Line Items]
Derivative Asset, Noncurrent		30	22
Exelon Generation Co L L C [Member] | Proprietary Trading [Member] | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative Liability, Current		(104)	(94)
Exelon Generation Co L L C [Member] | Proprietary Trading [Member] | Other Noncurrent Liabilities [Member]
Derivative [Line Items]
Mark-to-market derivative liabilities		(40)	(33)
Exelon Generation Co L L C [Member] | Collateral And Netting [Member]
Derivative [Line Items]
Total mark-to-market derivative net assets (liabilities)	[2]	1,039	1,234
Total mark-to-market derivative assets	[2]	(6,411)	(5,157)
Total mark-to-market derivative liabilities	[2]	7,450	6,391
Exelon Generation Co L L C [Member] | Collateral And Netting [Member] | Other Current Assets [Member]
Derivative [Line Items]
Derivative Asset, Current		(4,792)	(3,994)
Exelon Generation Co L L C [Member] | Collateral And Netting [Member] | Other Noncurrent Assets [Member]
Derivative [Line Items]
Derivative Asset, Noncurrent		(1,619)	(1,163)
Exelon Generation Co L L C [Member] | Collateral And Netting [Member] | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative Liability, Current		5,505	4,827
Exelon Generation Co L L C [Member] | Collateral And Netting [Member] | Other Noncurrent Liabilities [Member]
Derivative [Line Items]
Mark-to-market derivative liabilities		1,945	1,564
Commonwealth Edison Co [Member]
Derivative [Line Items]
Derivative Liability, Current		(26)	(23)
Mark-to-market derivative liabilities		(239)	(224)
Commonwealth Edison Co [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Total mark-to-market derivative net assets (liabilities)	[3]	(265)	(247)
Total mark-to-market derivative assets	[3]	0	0
Total mark-to-market derivative liabilities	[3]	(265)	(247)
Commonwealth Edison Co [Member] | Designated as Hedging Instrument [Member] | Other Current Assets [Member]
Derivative [Line Items]
Derivative Asset, Current		0	0
Commonwealth Edison Co [Member] | Designated as Hedging Instrument [Member] | Other Noncurrent Assets [Member]
Derivative [Line Items]
Derivative Asset, Noncurrent		0	0
Commonwealth Edison Co [Member] | Designated as Hedging Instrument [Member] | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative Liability, Current		26	(23)
Commonwealth Edison Co [Member] | Designated as Hedging Instrument [Member] | Other Noncurrent Liabilities [Member]
Derivative [Line Items]
Mark-to-market derivative liabilities		239	(224)
Delmarva Power and Light Company [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Total mark-to-market derivative net assets (liabilities)		(1)	(2)
Total mark-to-market derivative assets		0	0
Total mark-to-market derivative liabilities		1	2
Delmarva Power and Light Company [Member] | Designated as Hedging Instrument [Member] | Other Current Assets [Member]
Derivative [Line Items]
Derivative Asset, Current		0	0
Delmarva Power and Light Company [Member] | Designated as Hedging Instrument [Member] | Other Noncurrent Assets [Member]
Derivative [Line Items]
Derivative Asset, Noncurrent		0	0
Delmarva Power and Light Company [Member] | Designated as Hedging Instrument [Member] | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative Liability, Current		1	2
Delmarva Power and Light Company [Member] | Designated as Hedging Instrument [Member] | Other Noncurrent Liabilities [Member]
Derivative [Line Items]
Mark-to-market derivative liabilities		0	0
Delmarva Power and Light Company [Member] | Derivative [Member]
Derivative [Line Items]
Total mark-to-market derivative net assets (liabilities)		0	0
Total mark-to-market derivative assets	[4]	0	0
Total mark-to-market derivative liabilities	[4]	0	0
Delmarva Power and Light Company [Member] | Derivative [Member] | Other Current Assets [Member]
Derivative [Line Items]
Derivative Asset, Current	[4]	0	0
Delmarva Power and Light Company [Member] | Derivative [Member] | Other Noncurrent Assets [Member]
Derivative [Line Items]
Derivative Asset, Noncurrent	[4]	0	0
Delmarva Power and Light Company [Member] | Derivative [Member] | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative Liability, Current	[4]	0	0
Delmarva Power and Light Company [Member] | Derivative [Member] | Other Noncurrent Liabilities [Member]
Derivative [Line Items]
Mark-to-market derivative liabilities	[4]	0	0
Delmarva Power and Light Company [Member] | Collateral And Netting [Member]
Derivative [Line Items]
Total mark-to-market derivative net assets (liabilities)		1	2
Total mark-to-market derivative assets		0	0
Total mark-to-market derivative liabilities		(1)	(2)
Delmarva Power and Light Company [Member] | Collateral And Netting [Member] | Other Current Assets [Member]
Derivative [Line Items]
Derivative Asset, Current		0	0
Delmarva Power and Light Company [Member] | Collateral And Netting [Member] | Other Noncurrent Assets [Member]
Derivative [Line Items]
Derivative Asset, Noncurrent		0	0
Delmarva Power and Light Company [Member] | Collateral And Netting [Member] | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative Liability, Current		(1)	(2)
Delmarva Power and Light Company [Member] | Collateral And Netting [Member] | Other Noncurrent Liabilities [Member]
Derivative [Line Items]
Mark-to-market derivative liabilities		0	0
PEPCO Holdings Inc [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
Mark-to-market derivative liabilities		0
Total mark-to-market derivative liabilities		$ 0
PHI PEPCO [Member] | Derivative [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
Mark-to-market derivative liabilities			0
Total mark-to-market derivative liabilities			0
PEPCO Holdings Inc [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
Mark-to-market derivative liabilities			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.