Fair Value of Financial Assets and Liabilities (Fair Value Assets Liabilities Measured On Recurring Basis Unobservable Input Reconciliation) (USD $)	3 Months Ended				6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011		Jun. 30, 2012		Jun. 30, 2011
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Purchases Sales Issuances Settlements [Abstract]
Purchases			$ (9,000,000)				$ (15,000,000)
Fair Value Inputs Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Abstract]
Beginning balance	428,000,000		82,000,000		67,000,000		50,000,000
Total realized / unrealized gains (losses)
Included in income	(67,000,000)		21,000,000		18,000,000		8,000,000
Included in other comprehensive income			(3,000,000)				(12,000,000)
Included in payable for Zion Station decommissioning	(1,000,000)		3,000,000		(1,000,000)		3,000,000
Included in regulatory assets	30,000,000		(85,000,000)		(5,000,000)		(33,000,000)
Change in collateral	4,000,000		2,000,000		(19,000,000)		7,000,000
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Purchases Sales Issuances Settlements [Abstract]
Purchases	70,000,000	[1]	17,000,000		406,000,000		48,000,000
Sales	(8,000,000)		(12,000,000)		(9,000,000)		(12,000,000)
Transfers into Level 3 - (Asset) / Liability	(34,000,000)				(34,000,000)
Transfers out of Level 3 - (Asset) / Liability	3,000,000		(7,000,000)		2,000,000		(41,000,000)
Ending balance	425,000,000		18,000,000		425,000,000		18,000,000
The amount of total gains (losses) included in income attributed to the change in unrealized gains (losses) related to assets and liabilities	(1,000,000)		30,000,000		103,000,000		23,000,000
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	(66,000,000)				(85,000,000)
Fair value of Constellation fair value assets acquired	310,000,000				310,000,000
Acquisition of marketable securities	14,000,000				14,000,000
Nuclear Decommissioning Trust Fund Investment Level Three Asset [Member] | Fair Value Inputs Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Abstract]
Beginning balance	13,000,000				13,000,000
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Purchases Sales Issuances Settlements [Abstract]
Purchases	41,000,000	[1]			41,000,000
Sales	0
Ending balance	54,000,000				54,000,000
The amount of total gains (losses) included in income attributed to the change in unrealized gains (losses) related to assets and liabilities	0				0
Pledged Assets For Zion Station Decommissioning [Member] | Fair Value Inputs Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Abstract]
Beginning balance	42,000,000		31,000,000		37,000,000
Total realized / unrealized gains (losses)
Included in payable for Zion Station decommissioning	(1,000,000)		3,000,000		(1,000,000)		3,000,000
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Purchases Sales Issuances Settlements [Abstract]
Purchases	26,000,000	[1]	12,000,000		32,000,000		43,000,000
Sales	(8,000,000)		(12,000,000)		(9,000,000)		(12,000,000)
Ending balance	59,000,000		34,000,000		59,000,000		34,000,000
The amount of total gains (losses) included in income attributed to the change in unrealized gains (losses) related to assets and liabilities	0				0
Derivative [Member] | Fair Value Inputs Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Abstract]
Beginning balance	359,000,000		51,000,000		17,000,000		50,000,000
Total realized / unrealized gains (losses)
Included in income	(67,000,000)	[2]	21,000,000	[3]	18,000,000	[2]	8,000,000	[3]
Included in other comprehensive income			(3,000,000)	[4]			(12,000,000)	[4]
Included in regulatory assets	30,000,000	[5]	(85,000,000)		(5,000,000)	[5]	(33,000,000)
Change in collateral	4,000,000		2,000,000		(19,000,000)		7,000,000
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Purchases Sales Issuances Settlements [Abstract]
Purchases	0	[1]	5,000,000		316,000,000	[1]	5,000,000
Sales	0
Transfers into Level 3 - (Asset) / Liability	(34,000,000)				(34,000,000)
Transfers out of Level 3 - (Asset) / Liability	3,000,000		(7,000,000)		2,000,000		(41,000,000)
Ending balance	295,000,000		(16,000,000)		295,000,000		(16,000,000)
The amount of total gains (losses) included in income attributed to the change in unrealized gains (losses) related to assets and liabilities	(1,000,000)		30,000,000		103,000,000		23,000,000
Other Investments [Member] | Fair Value Inputs Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Abstract]
Beginning balance	14,000,000				0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Purchases Sales Issuances Settlements [Abstract]
Purchases	3,000,000	[1]			17,000,000
Sales	0
Transfers out of Level 3 - (Asset) / Liability	0
Ending balance	17,000,000				17,000,000
The amount of total gains (losses) included in income attributed to the change in unrealized gains (losses) related to assets and liabilities	0				0
Exelon Generation Co L L C [Member]
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Purchases Sales Issuances Settlements [Abstract]
Purchases			(9,000,000)				(15,000,000)
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Abstract]
Increase (decrease) in fair value related to the swap contract			(65,000,000)				2,000,000
Realized gains (losses) related to swap contract			(108,000,000)				(220,000,000)
Increase (decrease) in fair value related to block contracts			(2,000,000)				(3,000,000)
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Abstract]
Beginning balance	1,251,000,000		964,000,000		867,000,000		1,030,000,000
Total realized / unrealized gains (losses)
Included in income	(71,000,000)		21,000,000		3,000,000		8,000,000
Included in other comprehensive income	(172,000,000)		(178,000,000)		(173,000,000)		(233,000,000)
Included in payable for Zion Station decommissioning	(1,000,000)		3,000,000		(1,000,000)		3,000,000
Change in collateral	4,000,000		2,000,000		(19,000,000)		7,000,000
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Purchases Sales Issuances Settlements [Abstract]
Purchases	70,000,000	[1]	17,000,000		406,000,000	[1]	48,000,000
Sales	(8,000,000)		(12,000,000)		(9,000,000)		(12,000,000)
Transfers into Level 3 - (Asset) / Liability	(34,000,000)				(34,000,000)
Transfers out of Level 3 - (Asset) / Liability	3,000,000		(7,000,000)		2,000,000		(41,000,000)
Ending balance	1,042,000,000		810,000,000		1,042,000,000		810,000,000
The amount of total gains (losses) included in income attributed to the change in unrealized gains (losses) related to assets and liabilities	(15,000,000)		30,000,000		78,000,000		23,000,000
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	(14,000,000)				(121,000,000)
Realized gains (losses) related to swap contract	(161,000,000)				(308,000,000)
Fair value of Constellation fair value assets acquired	310,000,000				310,000,000
Acquisition of marketable securities	14,000,000				14,000,000
Exelon Generation Co L L C [Member] | Nuclear Decommissioning Trust Fund Investment Level Three Asset [Member] | Fair Value Inputs Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Abstract]
Beginning balance	13,000,000		31,000,000		13,000,000
Total realized / unrealized gains (losses)
Included in payable for Zion Station decommissioning			3,000,000				3,000,000
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Purchases Sales Issuances Settlements [Abstract]
Purchases	41,000,000	[1]	12,000,000		41,000,000	[1]	43,000,000
Sales			(12,000,000)				(12,000,000)
Ending balance	54,000,000		34,000,000		54,000,000		34,000,000
Exelon Generation Co L L C [Member] | Pledged Assets For Zion Station Decommissioning [Member] | Fair Value Inputs Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Abstract]
Beginning balance	42,000,000				37,000,000
Total realized / unrealized gains (losses)
Included in payable for Zion Station decommissioning	(1,000,000)				(1,000,000)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Purchases Sales Issuances Settlements [Abstract]
Purchases	26,000,000	[1]			32,000,000	[1]
Sales	(8,000,000)				(9,000,000)
Ending balance	59,000,000				59,000,000
Exelon Generation Co L L C [Member] | Derivative [Member] | Fair Value Inputs Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Abstract]
Beginning balance	1,182,000,000		933,000,000		817,000,000		1,030,000,000
Total realized / unrealized gains (losses)
Included in income	(71,000,000)	[6]	21,000,000	[3]	3,000,000	[6]	8,000,000	[3]
Included in other comprehensive income	(172,000,000)	[7]	(178,000,000)	[8]	(173,000,000)	[7]	(233,000,000)	[8]
Change in collateral	4,000,000		2,000,000		(19,000,000)		7,000,000
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Purchases Sales Issuances Settlements [Abstract]
Purchases	0	[1]	5,000,000		316,000,000	[1]	5,000,000
Sales					0
Transfers into Level 3 - (Asset) / Liability	(34,000,000)				(34,000,000)
Transfers out of Level 3 - (Asset) / Liability	3,000,000		(7,000,000)		2,000,000		(41,000,000)
Ending balance	912,000,000		776,000,000		912,000,000		776,000,000
The amount of total gains (losses) included in income attributed to the change in unrealized gains (losses) related to assets and liabilities	(15,000,000)		30,000,000		78,000,000		23,000,000
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	(56,000,000)				(75,000,000)
Exelon Generation Co L L C [Member] | Other Investments [Member] | Fair Value Inputs Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Abstract]
Beginning balance	14,000,000
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Purchases Sales Issuances Settlements [Abstract]
Purchases	3,000,000	[1]			17,000,000	[1]
Sales					0
Transfers out of Level 3 - (Asset) / Liability					0
Ending balance	17,000,000				17,000,000
Commonwealth Edison Co [Member]
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Abstract]
Increase (decrease) in fair value related to the swap contract			65,000,000				(2,000,000)
Realized gains (losses) related to swap contract			108,000,000				220,000,000
Increase (decrease) in fair value related to block contracts			2,000,000				3,000,000
Increase (decrease) in fair value related to floating-to-fixed energy swap contracts	(30,000,000)		(86,000,000)		5,000,000		(35,000,000)
Commonwealth Edison Co [Member] | Fair Value Inputs Level 3 [Member]
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	14,000,000				121,000,000
Realized gains (losses) related to swap contract	161,000,000				308,000,000
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	(823,000,000)		(875,000,000)		(800,000,000)		(971,000,000)
Total realized / unrealized gains (losses)
Included in regulatory assets	(206,000,000)	[10],[9]	(87,000,000)	[11],[12]	(183,000,000)	[10],[9]	(183,000,000)	[11],[12]
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Purchases Sales Issuances Settlements [Abstract]
Ending balance	(617,000,000)		(788,000,000)		(617,000,000)		(788,000,000)
PECO Energy 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			(7,000,000)				(9,000,000)
Total realized / unrealized gains (losses)
Included in regulatory assets			(3,000,000)	[13]			(5,000,000)	[13]
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Purchases Sales Issuances Settlements [Abstract]
Ending balance			$ (4,000,000)				$ (4,000,000)

[1] Includes $310 million of fair value from contracts and $14 million of other investments acquired as a result of the merger.

[2] Includes the reclassification of $66 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 June 30, 2012.

[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] 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.

[5] Excludes $14 million and $121 million of decreases in fair value and $161 million and $308 million of realized losses due to settlements for the three and six months ended June 30, 2012 of Generation's financial swap contract with ComEd, which eliminates upon consolidation in Exelon's Consolidated Financial Statements

[6] Includes the reclassification of $56 million and $75 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, 2012, respectively.

[7] Includes $14 million of decreases in fair value and $121 million of increases in fair value and realized losses due to settlements of $161 million and $308 million associated with Generation's financial swap contract with ComEd for the three and six months ended June 30, 2012, respectively. This position was de-designated as a cash flow hedge prior to the merger date. All prospective changes in fair value and reclassifications of realized amounts are being recorded to income offset by the amortization of the frozen mark in OCI. All items eliminate upon consolidation in Exelon's Consolidated Financial Statements.

[8] 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.

[9] Includes $14 million of increases in fair value and $121 million of decreases in fair value and realized gains due to settlements of $161 million and $308 million associated with ComEd's financial swap contract with Generation for the three and six months ended June 30, 2012, respectively. All items eliminate upon consolidation in Exelon's Consolidated Financial Statements.

[10] Includes $30 million of decreases in fair value and $5 million of increases in the fair value of floating-to-fixed energy swap contracts with unaffiliated suppliers for the three and six months ended June 30, 2012, respectively.

[11] 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.

[12] 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.

[13] Includes increases of $2 million and $3 million related to the settlement of PECO's block contract with Generation for the three and six months ended June 30, 2011, respectively, which eliminate upon consolidation in Exelon's Consolidated Financial Statements.