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	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Abstract]
Beginning balance	$ 67.00
Total realized / unrealized gains (losses)
Included in income	(119.00)
Included in other comprehensive income	0
Included in regulatory assets	40.00
Change in collateral	(32.00)		6.00
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Purchases Sales Issuances Settlements [Abstract]
Purchases	583.00
Sales	(11.00)
Transfers out of Level 3 - (Asset) / Liability	89.00
Ending balance	656.00		67.00
The amount of total gains (losses) included in income attributed to the change in unrealized gains (losses) related to assets and liabilities	37.00
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	(156.00)
Fair Value Inputs Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Abstract]
Beginning balance	67.00		50.00
Total realized / unrealized gains (losses)
Included in income			100.00
Included in other comprehensive income			(25.00)
Included in regulatory assets			(104.00)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Purchases Sales Issuances Settlements [Abstract]
Purchases			80.00
Sales			(23.00)
Transfers into Level 3 - (Asset) / Liability	39.00
Transfers out of Level 3 - (Asset) / Liability			(17.00)
Ending balance			67.00
The amount of total gains (losses) included in income attributed to the change in unrealized gains (losses) related to assets and liabilities			132.00
Nuclear Decommissioning Trust Fund Investment Level Three Asset [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Abstract]
Beginning balance	13.00
Total realized / unrealized gains (losses)
Included in income	0
Included in other comprehensive income	0
Included in regulatory assets	1.00
Change in collateral	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Purchases Sales Issuances Settlements [Abstract]
Purchases	169.00
Sales	0
Transfers out of Level 3 - (Asset) / Liability	0
Ending balance	183.00
The amount of total gains (losses) included in income attributed to the change in unrealized gains (losses) related to assets and liabilities	0
Nuclear Decommissioning Trust Fund Investment Level Three Asset [Member] | Fair Value Inputs Level 3 [Member]
Total realized / unrealized gains (losses)
Included in income			1.00
Included in regulatory assets			2.00
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Purchases Sales Issuances Settlements [Abstract]
Purchases			10.00
Ending balance			13.00
The amount of total gains (losses) included in income attributed to the change in unrealized gains (losses) related to assets and liabilities			1.00
Pledged Assets For Zion Station Decommissioning [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Abstract]
Beginning balance	37.00
Total realized / unrealized gains (losses)
Included in income	0
Included in other comprehensive income	0
Included in regulatory assets	0
Change in collateral	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Purchases Sales Issuances Settlements [Abstract]
Purchases	63.00
Sales	(11.00)
Transfers out of Level 3 - (Asset) / Liability	0
Ending balance	89.00
The amount of total gains (losses) included in income attributed to the change in unrealized gains (losses) related to assets and liabilities	0
Pledged Assets For Zion Station Decommissioning [Member] | Fair Value Inputs Level 3 [Member]
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Purchases Sales Issuances Settlements [Abstract]
Purchases			60.00
Sales			(23.00)
Ending balance			37.00
Derivative [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Abstract]
Beginning balance	17.00
Total realized / unrealized gains (losses)
Included in income	(119.00)	[1]
Included in regulatory assets	39.00
Change in collateral	(32.00)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Purchases Sales Issuances Settlements [Abstract]
Purchases	334.00
Sales	0
Transfers out of Level 3 - (Asset) / Liability	89.00
Ending balance	367.00
The amount of total gains (losses) included in income attributed to the change in unrealized gains (losses) related to assets and liabilities	37.00
Derivative [Member] | Fair Value Inputs Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Abstract]
Beginning balance	17.00		50.00
Total realized / unrealized gains (losses)
Included in income			99.00
Included in other comprehensive income			(25.00)	[1]
Included in regulatory assets			(106.00)	[2]
Change in collateral			6.00
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Purchases Sales Issuances Settlements [Abstract]
Purchases			10.00
Transfers into Level 3 - (Asset) / Liability	39.00
Transfers out of Level 3 - (Asset) / Liability			(17.00)
Ending balance			17.00
The amount of total gains (losses) included in income attributed to the change in unrealized gains (losses) related to assets and liabilities			131.00
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Abstract]
Fair value of Constellation fair value assets acquired	323.00
Acquisition of marketable securities	17.00
Other Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Abstract]
Beginning balance	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Purchases Sales Issuances Settlements [Abstract]
Purchases	17.00
Ending balance	17.00
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	867.00		1,030.00
Total realized / unrealized gains (losses)
Included in income	(112.00)		(100.00)
Included in other comprehensive income	475.00		311.00
Included in payable for Zion Station decommissioning			(2.00)
Included in noncurrent payables to affiliates	(1.00)
Change in collateral	32.00		(6.00)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Purchases Sales Issuances Settlements [Abstract]
Purchases	583.00		80.00
Sales	11.00		23.00
Transfers into Level 3 - (Asset) / Liability	(39.00)
Transfers out of Level 3 - (Asset) / Liability	89.00		(17.00)
Ending balance	949.00		867.00
The amount of total gains (losses) included in income attributed to the change in unrealized gains (losses) related to assets and liabilities	(12.00)		(132.00)
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	13.00
Total realized / unrealized gains (losses)
Included in income			(1.00)
Included in payable for Zion Station decommissioning			(2.00)
Included in noncurrent payables to affiliates	(1.00)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Purchases Sales Issuances Settlements [Abstract]
Purchases	169.00		10.00
Ending balance	183.00		13.00
The amount of total gains (losses) included in income attributed to the change in unrealized gains (losses) related to assets and liabilities			(1.00)
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	37.00
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Purchases Sales Issuances Settlements [Abstract]
Purchases	63.00		60.00
Sales	11.00		23.00
Ending balance	89.00		37.00
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	817.00		1,030.00
Total realized / unrealized gains (losses)
Included in income	(112.00)	[1]	(99.00)
Included in other comprehensive income	475.00		311.00
Change in collateral	32.00		(6.00)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Purchases Sales Issuances Settlements [Abstract]
Purchases	334.00		10.00
Transfers into Level 3 - (Asset) / Liability	(39.00)
Transfers out of Level 3 - (Asset) / Liability	89.00		(17.00)
Ending balance	660.00		817.00
The amount of total gains (losses) included in income attributed to the change in unrealized gains (losses) related to assets and liabilities	(12.00)		(131.00)
Exelon Generation Co L L C [Member] | Other Investments [Member] | Fair Value Inputs Level 3 [Member]
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Purchases Sales Issuances Settlements [Abstract]
Purchases	17.00
Ending balance	17.00
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	(800.00)		(971.00)
Total realized / unrealized gains (losses)
Included in regulatory assets	507.00		171.00
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Purchases Sales Issuances Settlements [Abstract]
Ending balance	(293.00)		(800.00)
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			(9.00)
Total realized / unrealized gains (losses)
Included in regulatory assets			9.00
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Purchases Sales Issuances Settlements [Abstract]
Ending balance			$ 0

[1] Includes the reclassification of $156 million of realized losses due to settlement of derivative contracts recorded in results of operations for the year ended December 31, 2012.

[2] Excludes $98 million of increases in fair value and $566 million of realized losses due to settlements for the year ended December 31, 2012 of Generation's financial swap contract with ComEd, which eliminates upon consolidation in Exelon’s Consolidated Financial Statements. 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.