Fair Value of Financial Assets and Liabilities (Fair Value Assets And Liabilities Measured On Recurring Basis Gain Loss Included In Earnings) (Fair Value Inputs Level 3 [Member], USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 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	$ (96,000,000)	$ 10,000,000	$ (9,000,000)	$ 7,000,000
Change in the unrealized gains (losses) relating to assets and liabilities held	(22,000,000)	17,000,000	93,000,000	21,000,000
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	29,000,000	11,000,000	27,000,000	1,000,000
Change in the unrealized gains (losses) relating to assets and liabilities held	21,000,000	13,000,000	10,000,000	2,000,000
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	(101,000,000)	10,000,000	(25,000,000)	7,000,000
Change in the unrealized gains (losses) relating to assets and liabilities held	(36,000,000)	17,000,000	68,000,000	21,000,000
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	30,000,000	11,000,000	28,000,000	1,000,000
Change in the unrealized gains (losses) relating to assets and liabilities held	$ 21,000,000	$ 13,000,000	$ 10,000,000	$ 2,000,000