Derivative Financial Instruments (Fair Value Measurments) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				12 Months Ended
	Mar. 31, 2013		Mar. 31, 2012		Dec. 31, 2012		Dec. 31, 2011
Cash Flow Hedge Accumulated Other Comprehensive Income [Abstract]
Ineffective portion recognized in income	$ 0
Cash Flow Hedge Activity Impact [Abstract]
Cash flow hedge activity impact on pre-tax net income based on reclassification adjustment from accumulated other comprehensive income	99		173
Derivatives Fair Value [Line Items]
Mark-to-market derivative assets	666				938
Mark-to-market derivative assets (noncurrent assets)	706				937
Mark-to-market derivative liabilities (current liabilities)	(181)				(352)
Mark-to-market derivative liabilities (noncurrent liabilities)	(259)				(281)
Other Derivatives Not Designated As Hedging Instruments [Abstract]
Change in fair value	(329)		109
Reclassification to realized at settlement	(57)		(33)
Net mark-to-market gains (losses)	(386)		76
Operating Revenue [Member]
Proprietary Trading Activities [Abstract]
Change in fair value	(4)		2
Reclassification to realized at settlement	6		1
Net mark-to-market gains (losses)	2		3
Operating Revenue [Member] | Business Intersegment Eliminations [Member]
Other Derivatives Not Designated As Hedging Instruments [Abstract]
Change in fair value	7	[1]	11	[1]
Reclassification to realized at settlement	10	[1]	0	[1]
Net mark-to-market gains (losses)	17	[1]	11	[1]
Derivative [Member]
Derivatives Fair Value [Line Items]
Mark-to-market derivative assets	662				934
Mark-to-market derivative assets (noncurrent assets)	653				878
Total mark-to-market derivative assets	1,315				1,812
Mark-to-market derivative liabilities (current liabilities)	(180)				350
Mark-to-market derivative liabilities (noncurrent liabilities)	(232)
Total mark-to-market derivative liabilities	(412)				600
Total mark-to-market derivative net assets (liabilities)	903				1,212
Total Cash Flow Hedges [Member]
Cash Flow Hedge Accumulated Other Comprehensive Income [Abstract]
Accumulated OCI derivative gain - Beginning Balance	368		488		488
Effective portion of changes in fair value	(1)	[2]	317	[3]
Accumulated OCI derivative gain - Ending Balance	309		703
Footnotes To Cash Flow Hedge Accumulated Other Comprehensive Income [Abstract]
Net gain (loss) related to effective portion of changes in fair value of treasury rate locks	(3)		(12)
Total Cash Flow Hedges [Member] | Operating Revenue [Member]
Cash Flow Hedge Accumulated Other Comprehensive Income [Abstract]
Reclassifications from accumulated OCI to net income	(58)	[4]	(105)
Total Cash Flow Hedges [Member] | Purchased Power [Member]
Cash Flow Hedge Accumulated Other Comprehensive Income [Abstract]
Ineffective portion recognized in income			3
Exelon Generation Co L L C [Member]
Cash Flow Hedge Activity Impact [Abstract]
Net unrealized pre-tax gain (loss) on effective cash flow hedges	1,928
Net unrealized pre-tax gain (loss) on effective cash flow hedges related to swap contract	693
Expected reclassification from accumulated other comprehensive income to results of operations	404
Expected reclassification from accumulated other comprehensive income to results of operations related to fair value of swap contracts	95
Cash flow hedge activity impact on pre-tax net income based on reclassification adjustment from accumulated other comprehensive income	223		320
Change in cash flow hedge ineffectiveness	0		5
Cash flow hedge ineffectiveness adjustment to accumulated other comprehensive income			5
Derivatives Fair Value [Line Items]
Mark-to-market derivative assets	666				938
Mark-to-market derivative assets with affiliate (current assets)	85				226
Mark-to-market derivative assets (noncurrent assets)	694				924
Mark-to-market derivative liabilities (current liabilities)	(166)				(334)
Mark-to-market derivative liabilities (noncurrent liabilities)	(199)				(232)
Other Derivatives Not Designated As Hedging Instruments [Abstract]
Change in fair value	(336)		98
Reclassification to realized at settlement	(67)		(33)
Net mark-to-market gains (losses)	(403)		65
Exelon Generation Co L L C [Member] | Operating Revenue [Member]
Proprietary Trading Activities [Abstract]
Change in fair value	(4)		2
Reclassification to realized at settlement	6		1
Net mark-to-market gains (losses)	2		3
Exelon Generation Co L L C [Member] | Operating Revenue [Member]
Other Derivatives Not Designated As Hedging Instruments [Abstract]
Change in fair value	(485)		138
Reclassification to realized at settlement	(101)		(60)
Net mark-to-market gains (losses)	(586)		78
Exelon Generation Co L L C [Member] | Purchased Power [Member]
Other Derivatives Not Designated As Hedging Instruments [Abstract]
Change in fair value	149
Reclassification to realized at settlement	34
Net mark-to-market gains (losses)	183
Exelon Generation Co L L C [Member] | Purchased Power And Fuel [Member]
Other Derivatives Not Designated As Hedging Instruments [Abstract]
Change in fair value			(40)
Reclassification to realized at settlement			27
Net mark-to-market gains (losses)			(13)
Exelon Generation Co L L C [Member] | Derivative [Member]
Derivatives Fair Value [Line Items]
Mark-to-market derivative assets	662	[5]			934	[6]
Mark-to-market derivative assets with affiliate (current assets)	85	[5]			226	[6]
Mark-to-market derivative assets (noncurrent assets)	653	[5]			878	[6]
Total mark-to-market derivative assets	1,400	[5]			2,038	[6]
Mark-to-market derivative liabilities (current liabilities)	(165)	[5]			(332)	[6]
Mark-to-market derivative liabilities (noncurrent liabilities)	(172)	[5]			(201)	[6]
Total mark-to-market derivative liabilities	(337)	[5]			(533)	[6]
Total mark-to-market derivative net assets (liabilities)	1,063	[5]			1,505	[6]
Exelon Generation Co L L C [Member] | Designated as Hedging Instrument [Member]
Derivatives Fair Value [Line Items]
Mark-to-market derivative assets	2,475	[7]			2,883	[8]
Mark-to-market derivative assets with affiliate (current assets)	85	[7]			226	[8]
Mark-to-market derivative assets (noncurrent assets)	1,415	[7]			1,792	[8]
Total mark-to-market derivative assets	3,975	[7]			4,901	[8]
Mark-to-market derivative liabilities (current liabilities)	(2,218)	[7]			(2,419)	[8]
Mark-to-market derivative liabilities (noncurrent liabilities)	(1,005)	[7]			(1,080)	[8]
Total mark-to-market derivative liabilities	(3,223)	[7]			(3,499)	[8]
Total mark-to-market derivative net assets (liabilities)	752	[7]			1,402	[8]
Footnotes To Derivative Instruments In Statement Of Financial Position Fair Value [Abstract]
Current assets collateral offset	(98)				113
Noncurrent assets collateral offset	70				201
Current liabilities collateral offset	(99)				(214)
Noncurrent liabilities collateral offset	(106)				(131)
Total cash collateral received net of cash collateral posted					31
Exelon Generation Co L L C [Member] | Cash Flow Hedging [Member]
Derivatives Fair Value [Line Items]
Mark-to-market derivative assets (noncurrent assets)					724
Footnotes To Derivative Instruments In Statement Of Financial Position Fair Value [Abstract]
Fair value swap contract current asset	85				226
Fair value swap contract noncurrent asset					0
Noncurrent liability DOE interest rate swap	23				28
Exelon Generation Co L L C [Member] | Proprietary Trading [Member]
Derivatives Fair Value [Line Items]
Mark-to-market derivative assets	1,989				2,469
Mark-to-market derivative assets (noncurrent assets)	588
Total mark-to-market derivative assets	2,577				3,193
Mark-to-market derivative liabilities (current liabilities)	(1,946)				(2,432)
Mark-to-market derivative liabilities (noncurrent liabilities)	(553)				(689)
Total mark-to-market derivative liabilities	(2,499)				(3,121)
Total mark-to-market derivative net assets (liabilities)	78				72
Exelon Generation Co L L C [Member] | Collateral And Netting [Member]
Derivatives Fair Value [Line Items]
Mark-to-market derivative assets	(3,802)				(4,418)
Mark-to-market derivative assets (noncurrent assets)	(1,350)				(1,638)
Total mark-to-market derivative assets	(5,152)				(6,056)
Mark-to-market derivative liabilities (current liabilities)	3,999				4,519
Mark-to-market derivative liabilities (noncurrent liabilities)	1,386				1,568
Total mark-to-market derivative liabilities	5,385				6,087
Total mark-to-market derivative net assets (liabilities)	233				31
Footnotes To Derivative Instruments In Statement Of Financial Position Fair Value [Abstract]
Total cash collateral received net of cash collateral posted	233
Exelon Generation Co L L C [Member] | Energy Related Hedges [Member]
Cash Flow Hedge Accumulated Other Comprehensive Income [Abstract]
Accumulated OCI derivative gain - Beginning Balance	532	[10],[11],[12],[9]	925	[13],[14],[15]	925	[13],[14],[15]
Effective portion of changes in fair value	0	[16]	432	[16],[17]
Accumulated OCI derivative gain - Ending Balance	397	[10],[11],[12],[9]	1,166	[13],[14],[15],[18]	532	[10],[11],[12],[9]	925	[13],[14],[15]
Footnotes To Cash Flow Hedge Accumulated Other Comprehensive Income [Abstract]
Unrealized gain (loss) related to fair value of swap contract	58		419		133		420
Net gain (loss) related to effective portion of changes in fair value of swap contract			88
Net gain (loss) of reclassifications from accumulated OCI to net income related to the settlements of swap contract			(89)
Net gains (losses) related to interest rate swaps and treasury rate locks	(17)		12		20		(10)
Exelon Generation Co L L C [Member] | Energy Related Hedges [Member] | Operating Revenue [Member]
Cash Flow Hedge Accumulated Other Comprehensive Income [Abstract]
Reclassifications from accumulated OCI to net income	(135)	[19],[20]	(194)	[21],[22]
Ineffective portion recognized in income			3
Footnotes To Cash Flow Hedge Accumulated Other Comprehensive Income [Abstract]
Net gain (loss) of reclassifications from accumulated OCI to net income related to the settlements of swap contract	(75)
Commonwealth Edison Co [Member]
Derivatives Fair Value [Line Items]
Mark-to-market derivative liabilities (current liabilities)	(15)				(18)
Mark-to-market derivative liability with affiliate (current liability)	(85)				(226)
Mark-to-market derivative liabilities (noncurrent liabilities)	(60)				(49)
Commonwealth Edison Co [Member] | Derivative [Member]
Derivatives Fair Value [Line Items]
Mark-to-market derivative liabilities (current liabilities)					(18)	[8]
Mark-to-market derivative liability with affiliate (current liability)					(226)	[8]
Mark-to-market derivative liabilities (noncurrent liabilities)					(49)	[8]
Total mark-to-market derivative liabilities					(293)	[8]
Total mark-to-market derivative net assets (liabilities)					(293)	[8]
Commonwealth Edison Co [Member] | Designated as Hedging Instrument [Member]
Derivatives Fair Value [Line Items]
Mark-to-market derivative liabilities (current liabilities)	(15)	[23],[7]
Mark-to-market derivative liability with affiliate (current liability)	(85)	[23],[7]
Mark-to-market derivative liabilities (noncurrent liabilities)	(60)	[23],[7]
Total mark-to-market derivative liabilities	(160)	[23],[7]
Total mark-to-market derivative net assets (liabilities)	(160)	[23],[7]
Commonwealth Edison Co [Member] | Cash Flow Hedging [Member]
Footnotes To Derivative Instruments In Statement Of Financial Position Fair Value [Abstract]
Fair value swap contract current liability	85				226
Fair value swap contract noncurrent liability					0
Commonwealth Edison Co [Member] | Business Intersegment Eliminations [Member]
Derivatives Fair Value [Line Items]
Mark-to-market derivative assets with affiliate (current assets)	(85)	[7]			(226)	[8]
Total mark-to-market derivative assets	(85)	[7]			(226)	[8]
Mark-to-market derivative liability with affiliate (current liability)	85	[7]			226	[8]
Total mark-to-market derivative liabilities	85	[7]			226	[8]
Total mark-to-market derivative net assets (liabilities)	$ 0	[7]

[1] Prior to the merger, the five-year financial swap contract between Generation and ComEd was de-designated. As a result, all prospective changes in fair value are recorded to operating revenues and eliminated in consolidation.

[2] Includes $23 million of losses, net of taxes, related to the effective portion of changes in fair value of interest rate swaps and treasury rate locks.

[3] (d)        Includes $12 million of losses, net of taxes, related to the effective portion of changes in fair value of interest rate swaps and treasury rate locks.

[4] (d)          Includes $3 million of losses, net of taxes, related to the effective portion of changes in fair value of interest rate swaps and treasury rate locks.

[5]  These are not reflected in the table above. (c)        Current and noncurrent assets are shown net of collateral of $(98) million and $70 million, respectively, and current and noncurrent liabilities are shown net of collateral of $(99) million and $(106) million, respectively. The total cash collateral posted, net of cash collateral received and offset against mark-to-market assets and liabilities was $233 million at March 31, 2013.

[6] (c)        Current and noncurrent assets are shown net of collateral of $113 million and $201 million, respectively, and current and noncurrent liabilities are shown net of collateral of $ (214) million and $ (131) million, respectively. The total cash collateral received, net of cash collateral posted and offset against mark-to-market assets and liabilities was $31 million at December 31, 2012.

[7] (a)        Includes current assets for Generation and current liabilities for ComEd of $85 million related to the fair value of the five-year financial swap contract between Generation and ComEd, as described above.

[8] (a)        Includes current and noncurrent assets for Generation and current and noncurrent liabilities for ComEd of $226 million related to the fair value of the five-year financial swap contract between Generation and ComEd, as described above. For Generation excludes $28 million noncurrent liability relating to an interest rate swap in connection with a loan agreement to fund Antelope Valley as discussed above.

[9] Includes $##D<gfvswapnotYTD> million and $589 million of gains, net of taxes, related to the fair value of the five-year financial swap contract with ComEd, and $##D<gfvPeconotYTD> million and $3 million of gains, net of taxes, related to the fair value of the block contracts with PECO as of ##D<cyperiod> and ##D<pyend>.

[10] Excludes $##D<ginterestrateswaps> million of losses and $10 million of losses, net of taxes, related to interest rate swaps and treasury locks for the ##D<curmonth> months ended ##D<cyperiod> and year ended December 31, 2011, respectively.

[11] (a)        Includes $58 million and $133 million of gains, net of taxes, related to the fair value of the five-year financial swap contract with ComEd, as of March 31, 2013 and December 31, 2012, respectively.

[12] (c)        Excludes $16 million of losses and $20 million of losses net of taxes, related to interest rate swaps and treasury rate locks as of March 31, 2013 and December 31, 2012, respectively.

[13] Includes $681 million and $585 million of gains, net of taxes, related to the fair value of the five-year financial swap contract with ComEd, and $3 million and $1 million of gains, net of taxes, related to the fair value of the block contracts with PECO as of ##D<pyperiod> and ##D<pyend2years>, respectively.

[14] (a)        Includes $419 million and $420 million of gains, net of taxes, related to the fair value of the five-year financial swap contract with ComEd at March 31, 2012 and December 31, 2011, respectively.

[15] (c)        Excludes $12 million of gains and $10 million of losses, net of tax, related to interest rate swaps and treasury rate locks for the three months ended March 31, 2012 and the year ended December 31, 2011, respectively.

[16] Includes $##D<gfveffswapnotYTD> million of gains, net of taxes, related to the effective portion of changes in fair value of the five-year financial swap contract with ComEd through the date of de-designation prior to the merger .

[17] Includes a $235 million of gains, net of taxes, related to the effective portion of changes in fair value of the five-year financial swap contract with ComEd, and a $2 million of gains, net of taxes, of the effective portion of changes in fair value of the block contracts with PECO for the ##D<curmonth> months ended ##D<pyperiod>. During the second quarter of 2010 the block contracts with PECO were designated as normal sales. As such, the mark-to-market balance on Generation's Consolidated Balance Sheet will be amortized over the term of the contract.

[18] Excludes $2 million of gains, net of taxes, related to interest rate swaps settled.

[19] Includes $75 million of losses, net of taxes, reclassified from accumulated OCI to recognize gains in net income related to the settlements of the five-year financial swap contract with ComEd.

[20] Includes a $##D<grclaccociswapYTD> million loss, net of taxes, reclassified from accumulated OCI to recognize gains in net income related to the settlements of the five-year financial swap contract with ComEd and a $##D<grclaccocipecoYTD> million loss, net of taxes, reclassified from accumulated OCI to recognize gains in net income related to the fair value of the block contracts with PECO for the ##D<curmonth> months ended ##D<cyperiod>.

[21] Includes a $139 million loss, net of taxes, of reclassifications from accumulated OCI to recognize gains in net income related to the settlements of the five-year financial swap contract with ComEd for the ##D<curmonth> months ended ##D<pyperiod>

[22] (b)        Includes a $89 million of losses, net of taxes, reclassified from accumulated OCI to recognize gains in net income related to the settlements of the five-year financial swap contract with ComEd.

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