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 $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 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	$ (153.00)	$ 108.00
Change in the unrealized gains (losses) relating to assets and liabilities held	13.00	137.00
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	34.00	(8.00)
Change in the unrealized gains (losses) relating to assets and liabilities held	24.00	(5.00)
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	(146.00)	108.00
Change in the unrealized gains (losses) relating to assets and liabilities held	(25.00)	137.00
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	34.00	(8.00)
Change in the unrealized gains (losses) relating to assets and liabilities held	$ 13.00	$ (5.00)