Investment in Constellation Energy Nuclear Group, LLC (Details) (USD $)	1 Months Ended	3 Months Ended			7 Months Ended		9 Months Ended
	May 31, 2011	Sep. 30, 2012		Sep. 30, 2011	Sep. 30, 2012		Sep. 30, 2012		Sep. 30, 2011
Impact Of Transactions Under Agreements [Abstract]
Amortization of energy contract assets and liabilities							$ 731,000,000	[1]
Power Purchase Agreement [Member]
Impact Of Transactions Under Agreements [Abstract]
Increase (Decrease) in earnings		(282,000,000)
Amount receivable from (payable to) related to agreements with CENG		(86,000,000)			(86,000,000)		(86,000,000)
Power Service Agency Agreement [Member]
Impact Of Transactions Under Agreements [Abstract]
Increase (Decrease) in earnings		1,000,000	[2]		2,000,000	[2]
Amount receivable from (payable to) related to agreements with CENG		0			0		0
Amortization of energy contract assets and liabilities		2,000,000					2,000,000
Administrative Services Agreement [Member]
Impact Of Transactions Under Agreements [Abstract]
Increase (Decrease) in earnings		14,000,000			30,000,000
Amount receivable from (payable to) related to agreements with CENG		5,000,000			5,000,000		5,000,000
Exelon Generation Co L L C [Member]
Schedule of Equity Method Investments [Line Items]
Percentage of ownership interest in CENG (as a percent)		50.01%			50.01%		50.01%
Basis difference in investment in CENG		204,000,000			204,000,000		204,000,000
Related Party Transaction [Line Items]
Purchase of nuclear output by EDF (as a percent)					49.99%
Maximum cost of service per month					358,000
Impact Of Transactions Under Agreements [Abstract]
Increase (Decrease) in earnings		459,000,000		304,000,000			1,233,000,000		856,000,000
CENG issued an unsecured revolving promissory note to borrow up to an aggregate principal amount	63,000,000				43,000,000
Percentage of request for borrowings submitted to entity (as a percent)					50.00%
Percentage of request for borrowings submitted to entity's subsidiaries (as a percent)					50.00%
Amortization of energy contract assets and liabilities							811,000,000	[1]
Debt Instrument, Basis Spread on Variable Rate		2.50%			2.50%		2.50%
Exelon Generation Co L L C [Member] | Minimum [Member]
Related Party Transaction [Line Items]
Required purchases of power from CENG's nuclear plants not sold to third parties (as a percent)					85.00%
Exelon Generation Co L L C [Member] | Maximum [Member]
Related Party Transaction [Line Items]
Required purchases of power from CENG's nuclear plants not sold to third parties (as a percent)					90.00%
Exelon Generation Co L L C [Member] | Power Purchase Agreement [Member]
Impact Of Transactions Under Agreements [Abstract]
Increase (Decrease) in earnings		(282,000,000)			(541,000,000)
Amount receivable from (payable to) related to agreements with CENG		(86,000,000)			(86,000,000)		(86,000,000)
Exelon Generation Co L L C [Member] | Power Service Agency Agreement [Member]
Impact Of Transactions Under Agreements [Abstract]
Increase (Decrease) in earnings		1,000,000	[2]		2,000,000	[2]
Amount receivable from (payable to) related to agreements with CENG		0			0		0
Amortization of energy contract assets and liabilities		2,000,000					2,000,000
Exelon Generation Co L L C [Member] | Administrative Services Agreement [Member]
Impact Of Transactions Under Agreements [Abstract]
Increase (Decrease) in earnings		14,000,000			30,000,000
Amount receivable from (payable to) related to agreements with CENG		5,000,000			5,000,000		5,000,000
Exelon Generation Co L L C [Member] | CENG [Member]
Schedule of Equity Method Investments [Line Items]
CENG		58,000,000					53,000,000
Amortization of basis difference in CENG		(57,000,000)					(131,000,000)
Total equity investment earnings (losses) - CENG		$ 1,000,000					$ (78,000,000)

[1] Included in purchased power and fuel expense on the Registrants' Consolidated Statements of Operations and Comprehensive Income

[2]