Fair Value of Financial Assets and Liabilities (Fair Value Assets And Liabilities Measured On Recurring Basis Gain Loss Included In Earnings) (Details) (Fair Value Inputs Level 3 [Member], USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Operating Revenue [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Gain Loss Included In Earnings [Line Items]
Total gains (losses) included in income	$ (106,000,000)	$ (5,000,000)	$ (195,000,000)	$ 2,000,000
Change in the unrealized gains (losses) relating to assets and liabilities held	(48,000,000)	1,000,000	(45,000,000)	22,000,000
Purchased Fuel and Electric [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Gain Loss Included In Earnings [Line Items]
Total gains (losses) included in income	9,000,000	(3,000,000)	38,000,000	(2,000,000)
Change in the unrealized gains (losses) relating to assets and liabilities held	6,000,000	(6,000,000)	29,000,000	(4,000,000)
Exelon Generation Co L L C [Member] | Operating Revenue [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Gain Loss Included In Earnings [Line Items]
Total gains (losses) included in income	(121,000,000)	(5,000,000)	(225,000,000)	2,000,000
Change in the unrealized gains (losses) relating to assets and liabilities held	(83,000,000)	1,000,000	(95,000,000)	22,000,000
Exelon Generation Co L L C [Member] | Purchased Fuel and Electric [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Gain Loss Included In Earnings [Line Items]
Total gains (losses) included in income	9,000,000	(3,000,000)	38,000,000	(2,000,000)
Change in the unrealized gains (losses) relating to assets and liabilities held	$ 6,000,000	$ (6,000,000)	$ 18,000,000	$ (4,000,000)