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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
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	$ 327	$ 171	$ 483	$ 125
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	(222)	(18)	(107)	173
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	(305)	(25)	(370)	122
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	(216)	99	(50)	368
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	(13)	(148)	(104)	(254)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	3	(119)	(48)	(202)
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	(12)	0	(10)	0
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	(12)	0	(10)	2
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	0	2	0	5
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	3	2	1	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	315	171	473	127
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	(210)	(18)	(97)	171
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	(305)	(25)	(370)	122
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	(216)	99	(50)	368
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	(13)	(148)	(104)	(254)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	3	(119)	(48)	(202)
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	0	2	0	5
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	3	2	1	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	(12)	0	(10)	0
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	(12)	0	(10)	2
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	(12)	0	(10)	0
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	$ (12)	$ 0	$ (10)	$ 2