Fair Value of Financial Assets and Liabilities (Fair Value Assets Liabilities Measured On Recurring Basis Unobservable Input Reconciliation) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended							3 Months Ended																		3 Months Ended
	Mar. 31, 2013 Fair Value Inputs Level 3 [Member]		Mar. 31, 2012 Fair Value Inputs Level 3 [Member]	Mar. 31, 2013 Nuclear Decommissioning Trust Fund Investment Level Three Asset [Member] Fair Value Inputs Level 3 [Member]		Mar. 31, 2012 Nuclear Decommissioning Trust Fund Investment Level Three Asset [Member] Fair Value Inputs Level 3 [Member]	Dec. 31, 2011 Nuclear Decommissioning Trust Fund Investment Level Three Asset [Member] Fair Value Inputs Level 3 [Member]	Mar. 31, 2013 Pledged Assets For Zion Station Decommissioning [Member] Fair Value Inputs Level 3 [Member]		Mar. 31, 2012 Pledged Assets For Zion Station Decommissioning [Member] Fair Value Inputs Level 3 [Member]	Mar. 31, 2013 Derivative [Member] Fair Value Inputs Level 3 [Member]		Mar. 31, 2012 Derivative [Member] Fair Value Inputs Level 3 [Member]		Mar. 31, 2013 Other Investments [Member] Fair Value Inputs Level 3 [Member]		Mar. 31, 2012 Other Investments [Member] Fair Value Inputs Level 3 [Member]	Mar. 31, 2013 Exelon Generation Co L L C [Member]	Mar. 31, 2013 Exelon Generation Co L L C [Member] Fair Value Inputs Level 3 [Member]		Mar. 31, 2012 Exelon Generation Co L L C [Member] Fair Value Inputs Level 3 [Member]	Mar. 31, 2013 Exelon Generation Co L L C [Member] Nuclear Decommissioning Trust Fund Investment Level Three Asset [Member] Fair Value Inputs Level 3 [Member]		Mar. 31, 2012 Exelon Generation Co L L C [Member] Nuclear Decommissioning Trust Fund Investment Level Three Asset [Member] Fair Value Inputs Level 3 [Member]	Dec. 31, 2011 Exelon Generation Co L L C [Member] Nuclear Decommissioning Trust Fund Investment Level Three Asset [Member] Fair Value Inputs Level 3 [Member]	Mar. 31, 2013 Exelon Generation Co L L C [Member] Pledged Assets For Zion Station Decommissioning [Member] Fair Value Inputs Level 3 [Member]		Mar. 31, 2012 Exelon Generation Co L L C [Member] Pledged Assets For Zion Station Decommissioning [Member] Fair Value Inputs Level 3 [Member]	Mar. 31, 2013 Exelon Generation Co L L C [Member] Derivative [Member] Fair Value Inputs Level 3 [Member]		Mar. 31, 2012 Exelon Generation Co L L C [Member] Derivative [Member] Fair Value Inputs Level 3 [Member]		Mar. 31, 2013 Exelon Generation Co L L C [Member] Other Investments [Member] Fair Value Inputs Level 3 [Member]	Mar. 31, 2012 Exelon Generation Co L L C [Member] Other Investments [Member] Fair Value Inputs Level 3 [Member]	Mar. 31, 2013 Commonwealth Edison Co [Member] Derivative [Member] Fair Value Inputs Level 3 [Member]		Mar. 31, 2012 Commonwealth Edison Co [Member] Derivative [Member] Fair Value Inputs Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Abstract]
Beginning balance	$ 656		$ 67	$ 183		$ 13	$ 13	$ 89		$ 37	$ 367		$ 17		$ 17				$ 949		$ 867	$ 183		$ 13	$ 13	$ 89		$ 37	$ 660		$ 817		$ 17	$ 0	$ (293)		$ (800)
Total realized / unrealized gains (losses)
Included in income	126		(85)	(1)							127	[1],[2]	(85)	[3]					143		(74)	(1)							144	[4]	(74)	[5],[6]
Included in other comprehensive income			0										0	[7],[8]					124		1								124	[9]	1	[10],[11]
Included in payable for Zion Station decommissioning	0							0
Included in regulatory assets	7		35	(1)							8	[12],[13]	35						(1)			(1)							0						(133)	[14],[15],[16],[17]	23	[18],[19]
Change in collateral	33		(36)								33		(36)						33		(36)								33		(36)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Purchases Sales Issuances Settlements [Abstract]
Purchases	49	[20]	349	32	[20]			22	[20]	6	(5)	[20]	329		0	[20]	14		49	[21]	349	32	[21]			22	[21]	6	(5)		329			14
Sales	(26)		1	(7)				(7)		1	(4)				(8)				(26)		(1)	(7)				(7)		(1)	(4)				(8)
Transfers into Level 3 - (Asset) / Liability	4										4								4										4				(9)
Transfers out of Level 3 - (Asset) / Liability	0		(1)								0		(1)								(1)										(1)
Ending balance	583		428	210		13	13	104		42	260		359		9		14		743		1,251	210		13	13	104		42	420		1,182			14	(160)		(823)
The amount of total gains (losses) included in income attributed to the change in unrealized gains (losses) related to assets and liabilities	(78)		104	1							(79)		104						(85)		93	1							(86)		93
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Abstract]
Gain (loss) reclassified to results of operating due to the settlement of derivative contracts											(58)		(19)					(58)													(19)
Increase (decrease) in fair value related to the swap contract													135					(133)													135				8		(135)
Realized gains (losses) related to swap contract													(147)																		(147)				(133)		147
Realized gains (losses) related to swap contract with unaffiliated parties																																			(5)
Increase (decrease) in fair value related to floating-to-fixed energy swap contracts																																			(11)		35
Fair value of Constellation fair value assets acquired			323																		323
Acquisition of marketable securities			$ 14																		$ 14

[1] Includes the reclassification of $##D<grclrelloss> million and $85 million of realized losses due to the settlement of derivative contracts recorded in results of operations for the three and six months ended ##D<cyperiod>.

[2] ) Includes the reclassification of $48 million of realized losses due to the settlement of derivative contracts recorded in results of operations for the three months ended March 31, 2013, respectively.

[3] Includes the reclassification of $9 million and $15 million of realized losses due to the settlement of derivative contracts recorded in results of operations for the three and six months ended June 30, 2011, respectively.

[4] (a)        Includes the reclassification of $58 million of realized losses due to the settlement of derivative contracts recorded in results of operations.

[5] Includes the reclassification of $9 million and $15 million of realized losses due to the settlement of derivative contracts recorded in results of operations for the three and six months ended June 30, 2011, respectively.

[6] (a)        Includes the reclassification of $19 million of realized losses due to the settlement of derivative contracts recorded in results of operations.

[7] Excludes $65 million of decreases and $2 million of increases in fair value and $108 million and $220 million of realized losses due to settlements associated with Generation?s financial swap contract with ComEd and $2 million and $3 million of changes in the fair value of Generation?s block contracts with PECO for the three months and six months ended June 30, 2011, respectively. All items eliminate upon consolidation if Exelon?s Consolidated Financial Statements.

[8] b)        Includes $135 million of increases in fair value and $147 million of realized losses due to settlements during 2012 of Generation's financial swap contract with ComEd, which eliminates upon consolidation in Exelon's Consolidated Financial Statements.

[9] (b)        Includes $8 million of increases in fair value and $133 million of realized losses due to settlements during 2013 of Generation's financial swap contract with ComEd, which eliminates upon consolidation in Exelon's Consolidated Financial Statements.

[10] Includes $65 million of decreases in fair value and $2 million of increases in fair value realized losses reclassified from OCI due to settlements of $108 million and $220 million associated with Generation's financial swap contract with ComEd and $2 million and $3 million of decreases in fair value due to settlement of Generation's block contracts with PECO for the three and six months ended June 30, 2011, respectively. All items eliminate upon consolidation in Exelon's Consolidated Financial Statements.

[11] (b)        Includes $135 million of increases in fair value and $147 million of realized losses due to settlements during 2012 of Generation's financial swap contract with ComEd, which eliminates upon consolidation in Exelon's Consolidated Financial Statements.

[12] Excludes $##D<gcomedfvpretaxqtd> million and $121 million of decreases in fair value and $##D<gcomedreclassqtd> million and $308 million of realized losses due to settlements for the three and six months ended ##D<cyperiod> of Generation's financial swap contract with ComEd, which eliminates upon consolidation in Exelon's Consolidated Financial Statements

[13] ) Excludes increases in fair value of $8 million and realized losses reclassified due to settlements of $133 million associated with Generation's financial swap contract with ComEd for the three months ended March 31, 2013, respectively. All items eliminate upon consolidation in Exelon's Consolidated Financial Statements.

[14] Includes $##D<gcomedfvpretaxqtd> million of increases in fair value and $121 million of decreases in fair value and realized gains due to settlements of $##D<gcomedreclassqtd> million and $308 million associated with ComEd's financial swap contract with Generation for the ##D<threemonth> and ##D<curmonth> months ended ##D<cyperiod>, respectively. All items eliminate upon consolidation in Exelon's Consolidated Financial Statements.

[15] Includes $##D<CCYQTDENSWAPVAR> million of decreases in fair value and $##D<CCYYTDENSWAPVAR> million of increases in the fair value of floating-to-fixed energy swap contracts with unaffiliated suppliers for the ##D<threemonth> and ##D<curmonth> months ended ##D<cyperiod>, respectively.

[16] (a)        Includes $8 million of decreases in fair value and realized losses due to settlements of $133 million associated with ComEd's financial swap contract with Generation for the three months ended March 31, 2013. All items eliminate upon consolidation in Exelon's Consolidated Financial Statements.

[17] (b)        Includes $11 million of increases in the fair value and realized losses due to settlements of $3 million recorded in purchased power expense associated with floating-to-fixed energy swap contracts with unaffiliated suppliers for the three months ended March 31, 2013.

[18] Includes $65 million of increases in fair value and $2 million of decreases in fair value and $108 million and $220 million of realized gains due to settlements associated with ComEd?s financial swap contract with Generation for the three and six months ended June 30, 2011, respectively. All items eliminate upon consolidation in Exelon?s Consolidated Financial Statements.

[19] Includes $86 million and $35 million of decreases in fair value of floating-to-fixed energy swap contracts with unaffiliated suppliers for the three and six months ended June 30, 2011, respectively.

[20] Includes $##D<gfvacquired> million of fair value from contracts and $14 million of other investments acquired as a result of the merger.

[21] (c) Includes $10 million which Generation was paid to enter into out of the money purchase contracts.