Derivative Financial Instruments - Summary of Derivative Fair Value Balances (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Derivative [Line Items]
Mark-to-market derivative assets (current assets)	$ 1,279		$ 727
Mark-to-market derivative assets (noncurrent assets)	773		607
Mark-to-market derivative liabilities (current liabilities)	(234)		(159)
Mark-to-market derivative liabilities (noncurrent liabilities)	(403)		(300)
Noncurrent Liability Interest Rate Swap AVSR			0
Total cash collateral received net of cash collateral posted			144
Exelon Generation Co L L C [Member]
Derivative [Line Items]
Mark-to-market derivative assets (current assets)	1,276		727
Mark-to-market derivative assets (noncurrent assets)	771		600
Mark-to-market derivative liabilities (current liabilities)	(214)		(142)
Mark-to-market derivative liabilities (noncurrent liabilities)	(105)		(120)
Commonwealth Edison Co [Member]
Derivative [Line Items]
Mark-to-market derivative liabilities (current liabilities)	(20)		(17)
Mark-to-market derivative liabilities (noncurrent liabilities)	(187)		(176)
Proprietary Trading [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Mark-to-market derivative assets (current assets)	456		1,476
Mark-to-market derivative assets (noncurrent assets)	56		285
Total mark-to-market derivative assets	512		1,761
Mark-to-market derivative liabilities (current liabilities)	(468)		(1,410)
Mark-to-market derivative liabilities (noncurrent liabilities)	(64)		(293)
Total mark-to-market derivative liabilities	(532)		(1,703)
Total mark-to-market derivative net assets (liabilities)	(20)		58
Collateral And Netting [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Mark-to-market derivative assets (current assets)	(4,184)	[1]	(3,364)	[1]
Mark-to-market derivative assets (noncurrent assets)	(1,112)	[1]	(1,060)	[1]
Total mark-to-market derivative assets	(5,296)	[1]	(4,424)	[1]
Mark-to-market derivative liabilities (current liabilities)	5,200	[1]	3,292	[1]
Mark-to-market derivative liabilities (noncurrent liabilities)	1,502	[1]	988	[1]
Total mark-to-market derivative liabilities	6,702	[1]	4,280	[1]
Total mark-to-market derivative net assets (liabilities)	1,406	[1]	(144)	[1]
Derivative [Member]
Derivative [Line Items]
Mark-to-market derivative assets (current assets)	1,264		728
Mark-to-market derivative assets (noncurrent assets)	765		569
Total mark-to-market derivative assets	2,029		1,297
Mark-to-market derivative liabilities (current liabilities)	(235)		(158)
Mark-to-market derivative liabilities (noncurrent liabilities)	(289)		(285)
Total mark-to-market derivative liabilities	(524)		(443)
Total mark-to-market derivative net assets (liabilities)	1,505		854
Derivative [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Mark-to-market derivative assets (current assets)	1,264	[2]	728	[3]
Mark-to-market derivative assets (noncurrent assets)	765	[2]	569	[3]
Total mark-to-market derivative assets	2,029	[2]	1,297	[3]
Mark-to-market derivative liabilities (current liabilities)	(215)	[2]	(141)	[3]
Mark-to-market derivative liabilities (noncurrent liabilities)	(102)	[2]	(109)	[3]
Total mark-to-market derivative liabilities	(317)	[2]	(250)	[3]
Total mark-to-market derivative net assets (liabilities)	1,712	[2]	1,047	[3]
Designated as Hedging Instrument [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Mark-to-market derivative assets (current assets)	4,992		2,616
Mark-to-market derivative assets (noncurrent assets)	1,821		1,344
Total mark-to-market derivative assets	6,813		3,960
Mark-to-market derivative liabilities (current liabilities)	(4,947)		(2,023)
Mark-to-market derivative liabilities (noncurrent liabilities)	(1,540)		(804)
Total mark-to-market derivative liabilities	(6,487)		(2,827)
Total mark-to-market derivative net assets (liabilities)	326		1,133
Current assets collateral offset	(416)		(84)
Noncurrent assets collateral offset	(171)		(72)
Current liabilities collateral offset	(599)		(12)
Collateral Amount Offset Against Fair Value Of Derivative Current Noncurrent Liability	(220)
Total cash collateral received net of cash collateral posted	1,406
Designated as Hedging Instrument [Member] | Commonwealth Edison Co [Member]
Derivative [Line Items]
Mark-to-market derivative assets (current assets)	0		0
Mark-to-market derivative assets (noncurrent assets)	0	[4]	0	[4]
Total mark-to-market derivative assets	0	[4]	0
Mark-to-market derivative liabilities (current liabilities)	(20)	[4]	(17)	[4]
Mark-to-market derivative liabilities (noncurrent liabilities)	(187)	[4]	(176)	[4]
Total mark-to-market derivative liabilities	(207)	[4]	(193)	[4]
Total mark-to-market derivative net assets (liabilities)	$ (207)	[4]	$ (193)	[4]

[1] Exelon and Generation net all available amounts allowed under the derivative accounting guidance on the balance sheet. These amounts include unrealized derivative transactions with the same counterparty under legally enforceable master netting agreements and cash collateral. In some cases Exelon and Generation may have other offsetting exposures, subject to a master netting or similar agreement, such as trade receivables and payables, transactions that do not qualify as derivatives, letters of credit and other forms of non-cash collateral. These are not reflected in the table above.

[2] Current and noncurrent assets are shown net of collateral of $(416) million and $(171) million, respectively, and current and noncurrent liabilities are shown net of collateral of $(599) million and $(220) million, respectively. The total cash collateral posted, net of cash collateral received and offset against mark-to-market assets and liabilities was $1,406 million at December 31, 2014.

[3] Current and noncurrent assets are shown net of collateral of $84 million and $72 million, respectively. Current liabilities are shown net of collateral of $(12) million. Collateral related to noncurrent liabilities was $0 million. The total cash collateral posted, net of cash collateral received and offset against mark-to-market assets and liabilities was $144 million at December 31, 2013.

[4] Includes current and noncurrent liabilities relating to floating-to-fixed energy swap contracts with unaffiliated suppliers.