Fair Value of Financial Assets and Liabilities (Fair Value Assets Liabilities Measured On Recurring Basis Unobservable Input Reconciliation) (Details) (USD $)	3 Months Ended		9 Months Ended		3 Months Ended			9 Months Ended		3 Months Ended			9 Months Ended	12 Months Ended		3 Months Ended			9 Months Ended		3 Months Ended				9 Months Ended				3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended	9 Months Ended		3 Months Ended			9 Months Ended			3 Months Ended	9 Months Ended	3 Months Ended		9 Months Ended		3 Months Ended	9 Months Ended	3 Months Ended				9 Months Ended				3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 Fair Value Inputs Level 3 [Member]		Sep. 30, 2011 Fair Value Inputs Level 3 [Member]	Sep. 30, 2012 Fair Value Inputs Level 3 [Member]	Sep. 30, 2011 Fair Value Inputs Level 3 [Member]	Sep. 30, 2012 Nuclear Decommissioning Trust Fund Investment Level Three Asset [Member]	Sep. 30, 2012 Nuclear Decommissioning Trust Fund Investment Level Three Asset [Member] Fair Value Inputs Level 3 [Member]		Sep. 30, 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]	Sep. 30, 2011 Nuclear Decommissioning Trust Fund Investment Level Three Asset [Member] Fair Value Inputs Level 3 [Member]	Sep. 30, 2012 Pledged Assets For Zion Station Decommissioning [Member] Fair Value Inputs Level 3 [Member]		Sep. 30, 2011 Pledged Assets For Zion Station Decommissioning [Member] Fair Value Inputs Level 3 [Member]	Sep. 30, 2012 Pledged Assets For Zion Station Decommissioning [Member] Fair Value Inputs Level 3 [Member]	Sep. 30, 2011 Pledged Assets For Zion Station Decommissioning [Member] Fair Value Inputs Level 3 [Member]	Sep. 30, 2012 Derivative [Member] Fair Value Inputs Level 3 [Member]		Sep. 30, 2011 Derivative [Member] Fair Value Inputs Level 3 [Member]		Sep. 30, 2012 Derivative [Member] Fair Value Inputs Level 3 [Member]		Sep. 30, 2011 Derivative [Member] Fair Value Inputs Level 3 [Member]		Sep. 30, 2012 Other Investments [Member] Fair Value Inputs Level 3 [Member]		Sep. 30, 2012 Other Investments [Member] Fair Value Inputs Level 3 [Member]	Sep. 30, 2011 Other Investments [Member] Fair Value Inputs Level 3 [Member]	Sep. 30, 2012 Exelon Generation Co L L C [Member] Fair Value Inputs Level 3 [Member]	Sep. 30, 2011 Exelon Generation Co L L C [Member] Fair Value Inputs Level 3 [Member]	Sep. 30, 2012 Exelon Generation Co L L C [Member] Fair Value Inputs Level 3 [Member]	Sep. 30, 2011 Exelon Generation Co L L C [Member] Fair Value Inputs Level 3 [Member]	Sep. 30, 2012 Exelon Generation Co L L C [Member] Nuclear Decommissioning Trust Fund Investment Level Three Asset [Member] Fair Value Inputs Level 3 [Member]	Sep. 30, 2011 Exelon Generation Co L L C [Member] Nuclear Decommissioning Trust Fund Investment Level Three Asset [Member] Fair Value Inputs Level 3 [Member]	Sep. 30, 2012 Exelon Generation Co L L C [Member] Nuclear Decommissioning Trust Fund Investment Level Three Asset [Member] Fair Value Inputs Level 3 [Member]	Sep. 30, 2011 Exelon Generation Co L L C [Member] Nuclear Decommissioning Trust Fund Investment Level Three Asset [Member] Fair Value Inputs Level 3 [Member]	Sep. 30, 2012 Exelon Generation Co L L C [Member] Pledged Assets For Zion Station Decommissioning [Member] Fair Value Inputs Level 3 [Member]	Sep. 30, 2012 Exelon Generation Co L L C [Member] Pledged Assets For Zion Station Decommissioning [Member] Fair Value Inputs Level 3 [Member]	Sep. 30, 2011 Exelon Generation Co L L C [Member] Pledged Assets For Zion Station Decommissioning [Member] Fair Value Inputs Level 3 [Member]	Sep. 30, 2012 Exelon Generation Co L L C [Member] Derivative [Member] Fair Value Inputs Level 3 [Member]	Sep. 30, 2011 Exelon Generation Co L L C [Member] Derivative [Member] Fair Value Inputs Level 3 [Member]		Sep. 30, 2012 Exelon Generation Co L L C [Member] Derivative [Member] Fair Value Inputs Level 3 [Member]	Sep. 30, 2011 Exelon Generation Co L L C [Member] Derivative [Member] Fair Value Inputs Level 3 [Member]		Sep. 30, 2012 Exelon Generation Co L L C [Member] Other Investments [Member] Fair Value Inputs Level 3 [Member]	Sep. 30, 2012 Exelon Generation Co L L C [Member] Other Investments [Member] Fair Value Inputs Level 3 [Member]	Sep. 30, 2012 Commonwealth Edison Co [Member]	Sep. 30, 2011 Commonwealth Edison Co [Member]	Sep. 30, 2012 Commonwealth Edison Co [Member]	Sep. 30, 2011 Commonwealth Edison Co [Member]	Sep. 30, 2011 Commonwealth Edison Co [Member] Fair Value Inputs Level 3 [Member]	Sep. 30, 2011 Commonwealth Edison Co [Member] Fair Value Inputs Level 3 [Member]	Sep. 30, 2012 Commonwealth Edison Co [Member] Derivative [Member] Fair Value Inputs Level 3 [Member]		Sep. 30, 2011 Commonwealth Edison Co [Member] Derivative [Member] Fair Value Inputs Level 3 [Member]		Sep. 30, 2012 Commonwealth Edison Co [Member] Derivative [Member] Fair Value Inputs Level 3 [Member]		Sep. 30, 2011 Commonwealth Edison Co [Member] Derivative [Member] Fair Value Inputs Level 3 [Member]		Sep. 30, 2011 PECO Energy Co [Member] Derivative [Member] Fair Value Inputs Level 3 [Member]		Sep. 30, 2011 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	$ 1,042,000,000				$ 425,000,000		$ 18,000,000	$ 67,000,000	$ 50,000,000		$ 54,000,000		$ 13,000,000		$ 6,000,000	$ 59,000,000		$ 34,000,000	$ 37,000,000		$ 295,000,000		$ (16,000,000)		$ 17,000,000		$ 50,000,000		$ 17,000,000		$ 0	$ (25,000,000)		$ 810,000,000		$ 1,030,000,000	$ 54,000,000	$ 34,000,000	$ 13,000,000		$ 59,000,000	$ 37,000,000		$ 912,000,000	$ 776,000,000		$ 817,000,000	$ 1,030,000,000		$ 17,000,000								$ (617,000,000)		$ (788,000,000)		$ (800,000,000)		$ (971,000,000)		$ (4,000,000)		$ (9,000,000)
Total realized / unrealized gains (losses)
Included in income	(112,000,000)		(187,000,000)		(97,000,000)		(8,000,000)	(157,000,000)	0												(97,000,000)	[1]	(8,000,000)	[2]	(157,000,000)	[1]	0	[2]						(8,000,000)		0								(112,000,000)	(8,000,000)	[2]	(187,000,000)	0	[2]
Included in other comprehensive income	(139,000,000)						(15,000,000)	1,000,000	(27,000,000)														(15,000,000)	[3]	1,000,000		(27,000,000)	[3]						(110,000,000)	(311,000,000)	(343,000,000)								(139,000,000)	(110,000,000)	[4]	(311,000,000)	(343,000,000)	[4]
Included in payable for Zion Station decommissioning	1,000,000		0		1,000,000		(3,000,000)	0	0							1,000,000		(3,000,000)	0	0														(3,000,000)		0		(3,000,000)			1,000,000	0	0
Included in regulatory assets					43,000,000		(18,000,000)	38,271,210.9219556	(51,000,000)	2,000,000			2,000,000								41,000,000	[5]	(18,000,000)		36,271,210.9219556		(51,000,000)																															(195,000,000)	[6],[7]	(76,000,000)	[8],[9]	(378,000,000)	[6],[7]	(259,000,000)	[8],[9]	(1,000,000)	[10]	(6,000,000)	[10]
Included in noncurrent payables to affiliates	(2,000,000)		(2,000,000)																														(2,000,000)		(2,000,000)
Change in collateral	(15,000,000)		(7,000,000)		(15,000,000)		8,000,000	(7,000,000)	15,000,000												(15,000,000)		8,000,000		(7,000,000)		15,000,000							8,000,000		15,000,000								(15,000,000)	8,000,000		(7,000,000)	15,000,000
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Purchases Sales Issuances Settlements [Abstract]
Purchases	18,000,000	6,000,000	437,000,000		18,000,000	[11]	23,000,000	437,000,000	70,000,000		14,000,000	[11]	55,000,000	6,000,000		4,000,000	[11]	17,000,000	36,000,000	60,000,000		[11]	0		329,000,000	[11]	4,000,000		0	[11]	17,000,000			23,000,000		70,000,000	14,000,000	17,000,000	55,000,000	6,000,000	4,000,000	36,000,000	60,000,000		0		329,000,000	4,000,000		0	17,000,000
Sales	0		(9,000,000)		0		(10,000,000)	(9,000,000)	(22,000,000)		0					0		(10,000,000)	(9,000,000)	(22,000,000)									0					(10,000,000)		(22,000,000)		(10,000,000)			0	(9,000,000)	(22,000,000)
Transfers into Level 3 - (Asset) / Liability	0		(34,000,000)		0			(34,000,000)																	(34,000,000)																						(34,000,000)
Transfers out of Level 3 - (Asset) / Liability	0		39,000,000	(16,000,000)	0		24,000,000	39,000,000	(16,000,000)														24,000,000		39,000,000		(16,000,000)		0					24,000,000		(16,000,000)									24,000,000		39,000,000
Ending balance	797,000,000		797,000,000		375,271,210.921956		19,000,000	375,271,210.921956	19,000,000		70,000,000		70,000,000	13,000,000	6,000,000	64,000,000		38,000,000	64,000,000	38,000,000	224,271,210.921956		(25,000,000)		224,271,210.921956		(25,000,000)		17,000,000		17,000,000	(25,000,000)	867,000,000	734,000,000	867,000,000	734,000,000	70,000,000	6,000,000	70,000,000	6,000,000	64,000,000	64,000,000	38,000,000	646,000,000	690,000,000		646,000,000	690,000,000		17,000,000	17,000,000							(422,000,000)		(712,000,000)		(422,000,000)		(712,000,000)		(3,000,000)		(3,000,000)
The amount of total gains (losses) included in income attributed to the change in unrealized gains (losses) related to assets and liabilities	(77,000,000)		(77,000,000)		(42,000,000)		(5,000,000)	(16,000,000)	18,000,000		0		0			0			0		(42,000,000)		(5,000,000)		(16,000,000)		18,000,000		0		0			(5,000,000)		18,000,000								(77,000,000)	(5,000,000)		(77,000,000)	18,000,000
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																					(55,000,000)		(9,000,000)		(141,000,000)		(15,000,000)																	(35,000,000)	(4,000,000)		(110,000,000)	(19,000,000)
Increase (decrease) in fair value related to the swap contract																																												(35,000,000)	(7,000,000)		86,000,000	4,000,000					7,000,000		4,000,000	(7,000,000)	4,000,000	35,000,000				(86,000,000)
Realized gains (losses) related to swap contract																																												(119,000,000)	(88,000,000)		(427,000,000)	(309,000,000)					88,000,000		309,000,000	(88,000,000)	(309,000,000)	119,000,000				427,000,000
Realized gains (losses) related to swap contract with unafilliated parties																																																				(1,000,000)		(2,000,000)
Increase (decrease) in fair value related to block contracts																																													(3,000,000)			(3,000,000)																		3,000,000		3,000,000
Increase (decrease) in fair value related to floating-to-fixed energy swap contracts																																																					(19,000,000)		(54,000,000)	(19,000,000)	(54,000,000)	(40,000,000)				33,000,000
Fair value of Constellation fair value assets acquired																					323,000,000				323,000,000								323,000,000		323,000,000
Acquisition of marketable securities																					$ 17,000,000				$ 17,000,000								$ 17,000,000		$ 17,000,000

[1] Includes the reclassification of $55 million and $141 million of realized losses due to the settlement of derivative contracts recorded in results of operations for the three and nine months ended September 30, 2012.

[2] Includes the reclassification of $4 million and $19 million of realized losses due to the settlement of derivative contracts recorded in results of operations for the three and nine months ended September 30, 2011, respectively.

[3] Excludes $7 million and $4 million of decreases in fair value and $88 million and $309 million of realized losses due to settlements associated with Generation’s financial swap contract with ComEd and $3 million of changes in the fair value of Generation’s block contracts with PECO for the nine months ended September 30, 2011. All items eliminate upon consolidation if Exelon’s Consolidated Financial Statements.

[4] Includes $7 million and $4 million of decreases in fair value realized losses reclassified from OCI due to settlements of $88 million and $309 million associated with Generation's financial swap contract with ComEd for the three and nine months ended September 30, 2011, and $3 million of decreases in fair value due to settlement of Generation's block contracts with PECO for the three and nine months ended September 30, 2011. All items eliminate upon consolidation in Exelon's Consolidated Financial Statements.

[5] Excludes $35 million of decreases in fair value and $86 million of increases in fair value and $119 million and $427 million of realized losses due to settlements for the three and nine months ended September 30, 2012 of Generation's financial swap contract with ComEd, which eliminates upon consolidation in Exelon's Consolidated Financial Statements

[6] Includes $35 million of increases in fair value and $86 million of decreases in fair value and realized gains due to settlements of $119 million and $427 million associated with ComEd's financial swap contract with Generation for the three and nine months ended September 30, 2012, respectively. All items eliminate upon consolidation in Exelon's Consolidated Financial Statements.

[7] Includes $40 million and $33 million of increases in the fair value and realized losses due to settlements of $1 million and $2 million recorded in purchased power expense associated with floating-to-fixed energy swap contracts with unaffiliated suppliers for the three and nine months ended September 30, 2012, respectively.

[8] Includes $7 million and $4 million of increases in fair value and $88 million and $309 million of realized gains due to settlements associated with ComEd's financial swap contract with Generation for the three and nine months ended September 30, 2011, respectively. All items eliminate upon consolidation in Exelon's Consolidated Financial Statements.

[9] Includes $19 million and $54 million of decreases in fair value of floating-to-fixed energy swap contracts with unaffiliated suppliers for the three and nine months ended September 30, 2011, respectively.

[10] Includes an increase of $3 million related to the settlement of PECO's block contract with Generation for the nine months ended September 30, 2011, which eliminate upon consolidation in Exelon's Consolidated Financial Statements.

[11] Includes $323 million of fair value from contracts and $17 million of other investments acquired as a result of the merger.