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		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 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	$ 166	$ (275)	$ 156	$ (46)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	(72)	339	115	191
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	(137)	275	(65)	147
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	(39)	360	166	269
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	(29)	(3)	(91)	(106)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	(33)	(23)	(51)	(83)
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	1	1	2	2
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	1	1	2	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	3
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	(1)	1	(2)	3
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	167	(274)	158	(44)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	(73)	338	113	189
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	(137)	275	(65)	147
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	(39)	360	166	269
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	(29)	(3)	(91)	(106)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	(33)	(23)	(51)	(83)
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	3
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	(1)	1	(2)	3
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	1	1	2	2
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	1	1	2	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	0	1	1	2
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	$ 0	$ 1	$ 1	$ 2