Derivative Financial Instruments (Fair Value Measurments) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Derivatives Fair Value [Line Items]
Mark-to-market derivative assets (current assets)	$ 934.00		$ 432.00
Mark-to-market derivative assets (noncurrent assets)	878.00		650.00
Total mark-to-market derivative assets	1,812.00		1,082.00
Mark-to-market derivative liabilities (current liabilities)	350.00		112.00
Mark-to-market derivative liability with affiliate (current liability)	0
Mark-to-market energy derivative liabilities (noncurrent liabilities)	250.00		107.00
Total mark-to-market derivative liabilities	600.00		219.00
Total mark-to-market derivative net assets (liabilities)	1,212.00		863.00
Other Derivatives Not Designated As Hedging Instruments [Abstract]
Change in fair value	(241.00)		218.00
Reclassification to realized at settlement	768.00		(515.00)
Net mark-to-market gains (losses)	527.00		(297.00)
Proprietary Trading Activities [Abstract]
Net mark-to-market gains (losses)			(3.00)
Pro Forma [Member]
Other Derivatives Not Designated As Hedging Instruments [Abstract]
Change in fair value	218.00		389.00
Reclassification to realized at settlement	(515.00)		(304.00)
Net mark-to-market gains (losses)	(297.00)		85.00
Total Cash Flow Hedges [Member]
Cash Flow Hedge Accumulated Other Comprehensive Income [Abstract]
Accumulated OCI derivative gain - Beginning Balance	488.00		400.00
Effective portion of changes in fair value	330.00	[1]	402.00	[1]
Accumulated OCI derivative gain - Ending Balance	368.00		488.00		400.00
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	747.00		512.00		754.00
Total Cash Flow Hedges [Member] | Operating Revenue [Member]
Cash Flow Hedge Accumulated Other Comprehensive Income [Abstract]
Reclassifications from accumulated OCI to net income	(453.00)		(309.00)
Ineffective portion recognized in income	3.00
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	(5.00)		10.00		1.00
Total Cash Flow Hedges [Member] | Purchased Power [Member]
Cash Flow Hedge Accumulated Other Comprehensive Income [Abstract]
Ineffective portion recognized in income			(5.00)
Exelon Generation Co L L C [Member]
Cash Flow Hedge Activity Impact [Abstract]
Net unrealized pre-tax gain (loss) on effective cash flow hedges	1,928.00
Net unrealized pre-tax gain (loss) on effective cash flow hedges related to swap contract	693.00
Expected reclassification from accumulated other comprehensive income to results of operations	684.00
Expected reclassification from accumulated other comprehensive income to results of operations related to fair value of swap contracts	219.00
Derivatives Fair Value [Line Items]
Mark-to-market derivative assets (current assets)	934.00		432.00
Mark-to-market derivative assets with affiliate (current assets)	226.00		503.00
Mark-to-market derivative assets (noncurrent assets)	878.00		635.00
Mark-to-market derivative assets with affiliate (noncurrent assets)	0		191.00
Total mark-to-market derivative assets	2,038.00		1,761.00
Mark-to-market derivative liabilities (current liabilities)	332.00		103.00
Mark-to-market derivative liability with affiliate (current liability)	0
Mark-to-market energy derivative liabilities (noncurrent liabilities)	201.00		10.00
Total mark-to-market derivative liabilities	533.00		113.00
Total mark-to-market derivative net assets (liabilities)	1,505.00		1,648.00
Other Derivatives Not Designated As Hedging Instruments [Abstract]
Change in fair value	(147.00)
Reclassification to realized at settlement	667.00
Net mark-to-market gains (losses)	520.00		(88.00)
Exelon Generation Co L L C [Member] | Operating Revenue [Member]
Proprietary Trading Activities [Abstract]
Change in fair value	(12.00)		23.00		26.00
Reclassification to realized at settlement	108.00		(26.00)		(24.00)
Net mark-to-market gains (losses)	96.00				2.00
Exelon Generation Co L L C [Member] | Operating Revenue [Member]
Other Derivatives Not Designated As Hedging Instruments [Abstract]
Change in fair value	(362.00)		87.00
Reclassification to realized at settlement	429.00		(296.00)
Net mark-to-market gains (losses)	67.00		(209.00)
Exelon Generation Co L L C [Member] | Operating Revenue [Member] | Pro Forma [Member]
Other Derivatives Not Designated As Hedging Instruments [Abstract]
Change in fair value	258.00
Reclassification to realized at settlement	(516.00)
Net mark-to-market gains (losses)	(258.00)
Exelon Generation Co L L C [Member] | Purchased Power And Fuel [Member]
Other Derivatives Not Designated As Hedging Instruments [Abstract]
Change in fair value	215.00		131.00
Reclassification to realized at settlement	238.00		(219.00)
Net mark-to-market gains (losses)	453.00
Exelon Generation Co L L C [Member] | Purchased Power And Fuel [Member] | Pro Forma [Member]
Other Derivatives Not Designated As Hedging Instruments [Abstract]
Change in fair value	(40.00)		389.00
Reclassification to realized at settlement	1.00		(304.00)
Net mark-to-market gains (losses)	(39.00)		85.00
Exelon Generation Co L L C [Member] | Designated as Hedging Instrument [Member]
Derivatives Fair Value [Line Items]
Mark-to-market derivative assets (current assets)	2,883.00		1,195.00
Mark-to-market derivative assets with affiliate (current assets)	226.00
Mark-to-market derivative assets (noncurrent assets)	1,792.00		582.00
Total mark-to-market derivative assets	4,901.00		1,777.00
Mark-to-market derivative liabilities (current liabilities)	2,419.00		965.00
Mark-to-market derivative liability with affiliate (current liability)	0
Mark-to-market energy derivative liabilities (noncurrent liabilities)	1,080.00		186.00
Total mark-to-market derivative liabilities	3,499.00		1,151.00
Total mark-to-market derivative net assets (liabilities)	1,402.00		626.00
Footnotes To Derivative Instruments In Statement Of Financial Position Fair Value [Abstract]
Current assets collateral offset	113.00		338.00
Noncurrent assets collateral offset	201.00		187.00
Current liabilities collateral offset	(214.00)		15.00
Noncurrent liabilities collateral offset	(131.00)
Total cash collateral received net of cash collateral posted			540.00
Exelon Generation Co L L C [Member] | Cash Flow Hedging [Member]
Derivatives Fair Value [Line Items]
Mark-to-market derivative assets (current assets)			438.00
Mark-to-market derivative assets with affiliate (current assets)			503.00
Mark-to-market derivative assets (noncurrent assets)			419.00
Mark-to-market derivative assets with affiliate (noncurrent assets)			191.00
Total mark-to-market derivative assets			1,551.00
Mark-to-market derivative liabilities (current liabilities)			9.00
Mark-to-market energy derivative liabilities (noncurrent liabilities)			4.00
Total mark-to-market derivative liabilities			13.00
Total mark-to-market derivative net assets (liabilities)			1,538.00
Footnotes To Derivative Instruments In Statement Of Financial Position Fair Value [Abstract]
Fair value swap contract current asset	226.00		503.00
Fair value swap contract noncurrent asset			191.00
Noncurrent liability DOE interest rate swap	28.00		19.00
Exelon Generation Co L L C [Member] | Proprietary Trading [Member]
Derivatives Fair Value [Line Items]
Mark-to-market derivative assets (current assets)	2,469.00		217.00
Mark-to-market derivative assets (noncurrent assets)	724.00		71.00
Total mark-to-market derivative assets	3,193.00		288.00
Mark-to-market derivative liabilities (current liabilities)	2,432.00		194.00
Mark-to-market energy derivative liabilities (noncurrent liabilities)	689.00		70.00
Total mark-to-market derivative liabilities	3,121.00		264.00
Total mark-to-market derivative net assets (liabilities)	72.00		24.00
Exelon Generation Co L L C [Member] | Collateral And Netting [Member]
Derivatives Fair Value [Line Items]
Mark-to-market derivative assets (current assets)	(4,418.00)		(1,418.00)
Mark-to-market derivative assets (noncurrent assets)	(1,638.00)		(437.00)
Total mark-to-market derivative assets	(6,056.00)		(1,855.00)
Mark-to-market derivative liabilities (current liabilities)	(4,519.00)		(1,065.00)
Mark-to-market energy derivative liabilities (noncurrent liabilities)	(1,568.00)		(250.00)
Total mark-to-market derivative liabilities	(6,087.00)		(1,315.00)
Total mark-to-market derivative net assets (liabilities)	31.00		(540.00)
Footnotes To Derivative Instruments In Statement Of Financial Position Fair Value [Abstract]
Total cash collateral received net of cash collateral posted	31.00
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	925.00	[2],[3]	1,011.00	[2],[3]
Effective portion of changes in fair value	432.00	[4]	504.00	[4]
Accumulated OCI derivative gain - Ending Balance	532.00	[2],[3]	925.00	[2],[3]	1,011.00	[2],[3]
Footnotes To Cash Flow Hedge Accumulated Other Comprehensive Income [Abstract]
Net gain (loss) related to effective portion of changes in fair value of swap contract	88.00		104.00
Net gains related to effective portion of changes in fair value of financial block contract					2.00
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	(828.00)	[5]	(585.00)	[5]
Ineffective portion recognized in income	3.00
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	375.00		273.00
Net gain (loss) of reclassifications from accumulated OCI to net income related to settlements of block contracts			3.00
Exelon Generation Co L L C [Member] | Energy Related Hedges [Member] | Purchased Power [Member]
Cash Flow Hedge Accumulated Other Comprehensive Income [Abstract]
Ineffective portion recognized in income			(5.00)
Exelon Generation Co L L C [Member] | Total Cash Flow Hedges [Member]
Footnotes To Cash Flow Hedge Accumulated Other Comprehensive Income [Abstract]
Net gains (losses) related to interest rate swaps and treasury rate locks			(20.00)		(10.00)
Net gain (loss) related to effective portion of changes in fair value of treasury rate locks	(9.00)		(12.00)
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	1,368.00		968.00		1,125.00
Exelon Generation Co L L C [Member] | Total Cash Flow Hedges [Member] | Operating Revenue [Member]
Cash Flow Hedge Activity Impact [Abstract]
Net unrealized pre-tax gain (loss) on effective cash flow hedges	(5.00)		10.00		1.00
Commonwealth Edison Co [Member]
Derivatives Fair Value [Line Items]
Mark-to-market derivative liability with affiliate (current liability)	(226.00)		(503.00)
Mark-to-market derivative liabilities with affiliate (noncurrent liabilities)	0		(191.00)
Commonwealth Edison Co [Member] | Designated as Hedging Instrument [Member]
Derivatives Fair Value [Line Items]
Mark-to-market derivative liabilities (current liabilities)	18.00		9.00
Mark-to-market derivative liability with affiliate (current liability)	226.00		503.00
Mark-to-market energy derivative liabilities (noncurrent liabilities)	49.00		97.00
Mark-to-market derivative liabilities with affiliate (noncurrent liabilities)			191.00
Total mark-to-market derivative liabilities	293.00		800.00
Total mark-to-market derivative net assets (liabilities)	(293.00)		(800.00)
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	226.00		503.00
Fair value swap contract noncurrent liability			191.00
PECO Energy Co [Member]
Footnotes To Cash Flow Hedge Accumulated Other Comprehensive Income [Abstract]
Amortization of realized loss on settled cash flow swaps	0		0		1.00
Derivatives Fair Value [Line Items]
Mark-to-market derivative liability with affiliate (current liability)	0		0
Mark-to-market derivative liabilities with affiliate (noncurrent liabilities)	0		0
Other Segment [Member] | Operating Revenue [Member] | Business Intersegment Eliminations [Member]
Other Derivatives Not Designated As Hedging Instruments [Abstract]
Change in fair value	(94.00)
Reclassification to realized at settlement	101.00
Net mark-to-market gains (losses)	7.00
Other Segment [Member] | Designated as Hedging Instrument [Member]
Derivatives Fair Value [Line Items]
Mark-to-market derivative assets (noncurrent assets)			15.00
Total mark-to-market derivative assets			15.00
Total mark-to-market derivative net assets (liabilities)			15.00
Other Segment [Member] | Business Intersegment Eliminations [Member]
Derivatives Fair Value [Line Items]
Mark-to-market derivative assets with affiliate (current assets)	(226.00)		(503.00)
Mark-to-market derivative assets with affiliate (noncurrent assets)			(191.00)
Total mark-to-market derivative assets	(226.00)		(694.00)
Mark-to-market derivative liability with affiliate (current liability)	(226.00)		(503.00)
Mark-to-market derivative liabilities with affiliate (noncurrent liabilities)			(191.00)
Total mark-to-market derivative liabilities	(226.00)		(694.00)
Total mark-to-market derivative net assets (liabilities)	$ 0

[1] Includes $9 million and $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 at Generation for the year ended December 31, 2012 and 2011, respectively

[2] Includes $133 million, $420 million and $589 million of gains, net of taxes, related to the fair value of the five-year financial swap contract with ComEd for the years ended December 31, 2012, 2011 and 2010, respectively, and $3 million of gains, net of taxes, related to the fair value of the block contracts with PECO for the year ended

[3] Excludes $20 million of losses and $10 million of losses, net of taxes, related to interest rate swaps and treasury rate locks for the years ended December 31, 2012 and 2011, respectively.

[4] Includes $88 million and $104 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 for the years ended December 31, 2012 and 2011, respectively, and $2 million of gains, net of taxes, of the effective portion of changes in fair value of the block contracts with PECO for the year ended

[5] Includes $375 million and $273 million of losses, net of taxes, reclassified from accumulated OCI to recognize gains in net income related to settlements of the five-year financial swap contract with ComEd for the years ended December 31, 2012 and 2011, respectively, and $3 million of losses, net of taxes, reclassified from accumulated OCI to recognize gains in net income related to settlements of the block contracts with PECO for the year ended