Derivative Financial Instruments - Summary of Gains and Losses on Hedges (Details) - Interest Expense [Member] - Fair Value Hedging [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Interest Rate Swap [Member]
Derivative [Line Items]
Gain (Loss) on Fair Value Hedges Recognized in Earnings	$ (13)	$ (9)	$ 3
Price Risk Derivative [Member]
Derivative [Line Items]
Increase (Decrease) in Fair Value of Hedged Item in Price Risk Fair Value Hedge	28	23	14
Exelon Generation Co L L C [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Gain (Loss) on Fair Value Hedges Recognized in Earnings	0	0	(1)	[1]
Exelon Generation Co L L C [Member] | Price Risk Derivative [Member]
Derivative [Line Items]
Increase (Decrease) in Fair Value of Hedged Item in Price Risk Fair Value Hedge	$ 0	$ 0	$ 0	[1]

[1] For the year ended December 31, 2015, the loss on Generation swaps included $(1) million realized in earnings with an immaterial amount excluded from hedge effectiveness testing.