Derivative Financial Instruments Derivative Financial Instruments - Summary of Derivative Fair Value Balances (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Derivative [Line Items]
Mark-to-market derivative assets (current assets)	$ 1,117		$ 1,279
Mark-to-market derivative assets (noncurrent assets)	913		773
Mark-to-market derivative liabilities (current liabilities)	(117)		(234)
Mark-to-market derivative liabilities (noncurrent liabilities)	(491)		(403)
Derivative [Member]
Derivative [Line Items]
Mark-to-market derivative assets (current assets)	1,102		1,264
Mark-to-market derivative assets (noncurrent assets)	892		765
Total mark-to-market derivative assets	1,994		2,029
Mark-to-market derivative liabilities (current liabilities)	(110)		(235)
Mark-to-market derivative liabilities (noncurrent liabilities)	(332)		(289)
Total mark-to-market derivative liabilities	(442)		(524)
Total mark-to-market derivative net assets (liabilities)	1,552		1,505
Exelon Generation Co L L C [Member]
Derivative [Line Items]
Mark-to-market derivative assets (current assets)	1,116		1,276
Mark-to-market derivative assets (noncurrent assets)	911		771
Mark-to-market derivative liabilities (current liabilities)	(97)		(214)
Mark-to-market derivative liabilities (noncurrent liabilities)	(121)		(105)
Exelon Generation Co L L C [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Mark-to-market derivative assets (current assets)	4,618		4,992
Mark-to-market derivative assets (noncurrent assets)	2,363		1,821
Total mark-to-market derivative assets	6,981		6,813
Mark-to-market derivative liabilities (current liabilities)	(4,505)		(4,947)
Mark-to-market derivative liabilities (noncurrent liabilities)	(2,016)		(1,540)
Total mark-to-market derivative liabilities	(6,521)		(6,487)
Total mark-to-market derivative net assets (liabilities)	460		326
Current assets collateral offset	387		416
Noncurrent assets collateral offset	192		171
Current liabilities collateral offset	519		599
Noncurrent liabilities collateral offset	248		220
Total cash collateral received net of cash collateral posted	1,346		(1,406)
Exelon Generation Co L L C [Member] | Derivative [Member]
Derivative [Line Items]
Mark-to-market derivative assets (current assets)	1,102	[1]	1,264	[2]
Mark-to-market derivative assets (noncurrent assets)	892	[1]	765	[2]
Total mark-to-market derivative assets	1,994	[1]	2,029	[2]
Mark-to-market derivative liabilities (current liabilities)	(90)	[1]	(215)	[2]
Mark-to-market derivative liabilities (noncurrent liabilities)	(111)	[1]	(102)	[2]
Total mark-to-market derivative liabilities	(201)	[1]	(317)	[2]
Total mark-to-market derivative net assets (liabilities)	1,793	[1]	1,712	[2]
Exelon Generation Co L L C [Member] | Proprietary Trading [Member]
Derivative [Line Items]
Mark-to-market derivative assets (current assets)	431		456
Mark-to-market derivative assets (noncurrent assets)	70		56
Total mark-to-market derivative assets	501		512
Mark-to-market derivative liabilities (current liabilities)	(437)		(468)
Mark-to-market derivative liabilities (noncurrent liabilities)	(77)		(64)
Total mark-to-market derivative liabilities	(514)		(532)
Total mark-to-market derivative net assets (liabilities)	(13)		(20)
Exelon Generation Co L L C [Member] | Collateral And Netting [Member]
Derivative [Line Items]
Mark-to-market derivative assets (current assets)	(3,947)		(4,184)
Mark-to-market derivative assets (noncurrent assets)	(1,541)		(1,112)
Total mark-to-market derivative assets	(5,488)	[3]	(5,296)	[3]
Mark-to-market derivative liabilities (current liabilities)	4,852		5,200
Mark-to-market derivative liabilities (noncurrent liabilities)	1,982		1,502
Total mark-to-market derivative liabilities	6,834	[3]	6,702	[3]
Total mark-to-market derivative net assets (liabilities)	1,346	[3]	1,406	[3]
Commonwealth Edison Co [Member]
Derivative [Line Items]
Mark-to-market derivative liabilities (current liabilities)	(20)		(20)
Mark-to-market derivative liabilities (noncurrent liabilities)	(221)		(187)
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)	(20)		(20)
Mark-to-market derivative liabilities (noncurrent liabilities)	(221)		(187)
Total mark-to-market derivative liabilities	(241)	[4]	(207)	[4]
Total mark-to-market derivative net assets (liabilities)	$ (241)	[4]	$ (207)	[4]

[1] Current and noncurrent assets are shown net of collateral of $387 million and $192 million, respectively, and current and noncurrent liabilities are shown net of collateral of $519 million and $248 million, respectively. The total cash collateral posted, net of cash collateral received and offset against mark-to-market assets and liabilities was $1,346 million at March 31, 2015.

[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] 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.