Derivative Financial Instruments - Summary of Proprietary Trading Activities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Unrealized Gain (Loss) on Derivatives	$ (151)	$ (24)	$ 367
Exelon Generation Co L L C [Member]
Derivative [Line Items]
Unrealized Gain (Loss) on Derivatives	(167)	(40)	249
Not Designated as Hedging Instrument, Trading [Member] | Exelon Generation Co L L C [Member] | Commodity Contract [Member] | Operating Revenue [Member]
Derivative [Line Items]
Unrealized Gain (Loss) on Derivatives	6	2	(6)
Not Designated as Hedging Instrument, Trading [Member] | Exelon Generation Co L L C [Member] | Commodity Contract [Member] | Operating Revenue [Member]
Derivative [Line Items]
Unrealized Gain (Loss) on Derivatives	$ (1)	$ (1)	$ (2)