Fair Value of Financial Assets and Liabilities - Fair Value Assets And Liabilities Measured On Recurring Basis Gain Loss Included In Earnings (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
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	$ 70	$ (39)	$ (260)	$ (61)
Change in the unrealized gains (losses) relating to assets and liabilities held	142	42	(293)	81
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	6	7	(24)	60
Change in the unrealized gains (losses) relating to assets and liabilities held	21	9	29	78
Other, net [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Gain Loss Included In Earnings [Line Items]
Total gains (losses) included in income	1	0	5	2
Change in the unrealized gains (losses) relating to assets and liabilities held	1	0	3	1
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	70	(39)	(260)	(67)
Change in the unrealized gains (losses) relating to assets and liabilities held	142	42	(293)	71
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	6	7	(24)	59
Change in the unrealized gains (losses) relating to assets and liabilities held	21	9	29	77
Exelon Generation Co L L C [Member] | Other, net [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Gain Loss Included In Earnings [Line Items]
Total gains (losses) included in income	1	0	5	2
Change in the unrealized gains (losses) relating to assets and liabilities held	$ 1	$ 0	$ 3	$ 1