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	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Fair Value Assets And Liabilities Measured On Recurring Basis Gain Loss Included In Earnings [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	$ 171	$ 259	$ 125	$ 186
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	(18)	(105)	173	154
Operating Revenue [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Gain Loss Included In Earnings [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(25)	(176)	122	(32)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	99	(64)	368	174
Purchased Power And Fuel [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Gain Loss Included In Earnings [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(148)	(83)	(254)	(156)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	(119)	(40)	(202)	(15)
Operating and Maintenance [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Gain Loss Included In Earnings [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	0	1	0	3
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	0	0	2	0
Other, net [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Gain Loss Included In Earnings [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	2	(1)	5	(1)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	2	(1)	5	(5)
Exelon Generation Co L L C [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Gain Loss Included In Earnings [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	171	260	127	189
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	(18)	(105)	171	154
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]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(25)	(176)	122	(32)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	99	(64)	368	174
Exelon Generation Co L L C [Member] | Purchased Power And Fuel [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Gain Loss Included In Earnings [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(148)	(83)	(254)	(156)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	(119)	(40)	(202)	(15)
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]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	2	(1)	5	(1)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	2	(1)	5	(5)
Pepco Holdings LLC [Member] | Operating and Maintenance [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Gain Loss Included In Earnings [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	0	1	0	3
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	0	0	2	0
Potomac Electric Power Company [Member] | Operating and Maintenance [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Gain Loss Included In Earnings [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	0	1	0	3
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	$ 0	$ 0	$ 2	$ 0