Derivative Financial Instruments Derivative Financial Instruments - Summary of Derivative Fair Value Balances (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Derivative [Line Items]
Mark-to-market derivative assets (current assets)	$ 744		$ 727
Mark-to-market derivative assets (noncurrent assets)	524		607
Mark-to-market derivative liabilities (current liabilities)	(249)		(159)
Mark-to-market derivative liabilities (noncurrent liabilities)	(291)		(300)
Derivative [Member]
Derivative [Line Items]
Mark-to-market derivative assets (current assets)	740		728
Mark-to-market derivative assets (noncurrent assets)	494		569
Total mark-to-market derivative assets	1,234		1,297
Mark-to-market derivative liabilities (current liabilities)	(243)		(158)
Mark-to-market derivative liabilities (noncurrent liabilities)	(265)		(285)
Total mark-to-market derivative liabilities	(508)		(443)
Total mark-to-market derivative net assets (liabilities)	726		854
Exelon Generation Co L L C [Member]
Derivative [Line Items]
Mark-to-market derivative assets (current assets)	744		727
Mark-to-market derivative assets (noncurrent assets)	507		600
Mark-to-market derivative liabilities (current liabilities)	(235)		(142)
Mark-to-market derivative liabilities (noncurrent liabilities)	(104)		(120)
Exelon Generation Co L L C [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Mark-to-market derivative assets (current assets)	3,230		2,616
Mark-to-market derivative assets (noncurrent assets)	1,372		1,344
Total mark-to-market derivative assets	4,602		3,960
Mark-to-market derivative liabilities (current liabilities)	(3,017)		(2,023)
Mark-to-market derivative liabilities (noncurrent liabilities)	(1,129)		(804)
Total mark-to-market derivative liabilities	(4,146)		(2,827)
Total mark-to-market derivative net assets (liabilities)	456		1,133
Current assets collateral offset	(96)		84
Noncurrent assets collateral offset	(50)		72
Current liabilities collateral offset	(205)		(12)
Noncurrent liabilities collateral offset	(108)		0
Total cash collateral received net of cash collateral posted	(459)		(144)
Exelon Generation Co L L C [Member] | Derivative [Member]
Derivative [Line Items]
Mark-to-market derivative assets (current assets)	740	[1]	728	[2]
Mark-to-market derivative assets (noncurrent assets)	494	[1]	569	[2]
Total mark-to-market derivative assets	1,234	[1]	1,297	[2]
Mark-to-market derivative liabilities (current liabilities)	(229)	[1]	(141)	[2]
Mark-to-market derivative liabilities (noncurrent liabilities)	(101)	[1]	(109)	[2]
Total mark-to-market derivative liabilities	(330)	[1]	(250)	[2]
Total mark-to-market derivative net assets (liabilities)	904	[1]	1,047	[2]
Exelon Generation Co L L C [Member] | Proprietary Trading [Member]
Derivative [Line Items]
Mark-to-market derivative assets (current assets)	752		1,476
Mark-to-market derivative assets (noncurrent assets)	128		285
Total mark-to-market derivative assets	880		1,761
Mark-to-market derivative liabilities (current liabilities)	(755)		(1,410)
Mark-to-market derivative liabilities (noncurrent liabilities)	(136)		(293)
Total mark-to-market derivative liabilities	(891)		(1,703)
Total mark-to-market derivative net assets (liabilities)	(11)		58
Exelon Generation Co L L C [Member] | Collateral And Netting [Member]
Derivative [Line Items]
Mark-to-market derivative assets (current assets)	(3,242)		(3,364)
Mark-to-market derivative assets (noncurrent assets)	(1,006)		(1,060)
Total mark-to-market derivative assets	(4,248)	[3]	(4,424)	[3]
Mark-to-market derivative liabilities (current liabilities)	3,543		3,292
Mark-to-market derivative liabilities (noncurrent liabilities)	1,164		988
Total mark-to-market derivative liabilities	4,707	[3]	4,280	[3]
Total mark-to-market derivative net assets (liabilities)	459	[3]	(144)	[3]
Commonwealth Edison Co [Member]
Derivative [Line Items]
Mark-to-market derivative liabilities (current liabilities)	(14)		(17)
Mark-to-market derivative liabilities (noncurrent liabilities)	(164)		(176)
Commonwealth Edison Co [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Mark-to-market derivative assets (current assets)	0		0
Mark-to-market derivative assets (noncurrent assets)	0		0
Total mark-to-market derivative assets	0	[4]	0	[4]
Mark-to-market derivative liabilities (current liabilities)	(14)		(17)
Mark-to-market derivative liabilities (noncurrent liabilities)	(164)		(176)
Total mark-to-market derivative liabilities	(178)	[4]	(193)	[4]
Total mark-to-market derivative net assets (liabilities)	$ (178)	[4]	$ (193)	[4]

[1] Current and noncurrent assets are shown net of collateral of $(96) million and $(50) million, respectively, and current and noncurrent liabilities are shown net of collateral of $(205) million and $(108) million, respectively. The total cash collateral posted, net of cash collateral received and offset against mark-to-market assets and liabilities was $459 million at September 30, 2014.

[2] Current and noncurrent assets are shown net of collateral of $84 million and $72 million, respectively, and current and noncurrent liabilities are shown net of collateral of $(12) million and $0 million, respectively. 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.

[3] 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.

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