Fair Value of Financial Assets and Liabilities - Fair Value Assets and Liabilities Measure on a Recurring Basis Gain Loss Included in Earnings (Details) - Fair Value, Inputs, Level 3 [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Fair Value Assets and Liabilities Measured on a Recurring Basis Gain Loss Included in Earnings [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	$ 409	$ 23
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	11	359
Operating Revenue [Member]
Fair Value Assets and Liabilities Measured on a Recurring Basis Gain Loss Included in Earnings [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(404)	219
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	(31)	546
Purchased Power And Fuel [Member]
Fair Value Assets and Liabilities Measured on a Recurring Basis Gain Loss Included in Earnings [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(10)	(245)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	37	(195)
Operating and Maintenance [Member]
Fair Value Assets and Liabilities Measured on a Recurring Basis Gain Loss Included in Earnings [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	3	3
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	3	3
Other, net [Member]
Fair Value Assets and Liabilities Measured on a 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	5
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	2	5
Exelon Generation Co L L C [Member]
Fair Value Assets and Liabilities Measured on a Recurring Basis Gain Loss Included in Earnings [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	412	26
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	8	356
Exelon Generation Co L L C [Member] | Operating Revenue [Member]
Fair Value Assets and Liabilities Measured on a Recurring Basis Gain Loss Included in Earnings [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(404)	219
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	(31)	546
Exelon Generation Co L L C [Member] | Purchased Power And Fuel [Member]
Fair Value Assets and Liabilities Measured on a Recurring Basis Gain Loss Included in Earnings [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(10)	(245)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	37	(195)
Exelon Generation Co L L C [Member] | Other, net [Member]
Fair Value Assets and Liabilities Measured on a 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	5
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	2	5
Pepco Holdings LLC [Member] | Operating and Maintenance [Member]
Fair Value Assets and Liabilities Measured on a Recurring Basis Gain Loss Included in Earnings [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	3	3
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	3	3
Potomac Electric Power Company [Member] | Operating and Maintenance [Member]
Fair Value Assets and Liabilities Measured on a Recurring Basis Gain Loss Included in Earnings [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	3	3
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	$ 3	$ 3