Fair Value of Financial Assets and Liabilities - Fair Value Assets and Liabilities Measure on Recurring Basis Gain Loss Included in Earnings (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Operating Revenue [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total gains (losses) included in income	$ 614		$ (152)
Change in the unrealized gains (losses) relating to assets and liabilities held	663		40
Purchased Fuel and Electric [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total gains (losses) included in income	(88)		108
Change in the unrealized gains (losses) relating to assets and liabilities held	(23)		127
Other, net [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total gains (losses) included in income	6	[1]	2	[1]
Change in the unrealized gains (losses) relating to assets and liabilities held	4	[1]	1	[1]
Exelon Generation Co L L C [Member] | Operating Revenue [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total gains (losses) included in income	614		(158)
Change in the unrealized gains (losses) relating to assets and liabilities held	663		30
Exelon Generation Co L L C [Member] | Purchased Fuel and Electric [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total gains (losses) included in income	(88)		107
Change in the unrealized gains (losses) relating to assets and liabilities held	(23)		126
Exelon Generation Co L L C [Member] | Other, net [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total gains (losses) included in income	6	[1]	2	[1]
Change in the unrealized gains (losses) relating to assets and liabilities held	$ 4	[1]	$ 1	[1]

[1] Other, net activity consists of realized and unrealized gains (losses) included in income for the NDT funds held by Generation.