Fair Value of Financial Assets and Liabilities - Fair Value Assets And Liabilities Measured On Recurring Basis Gain Loss Included In Earnings (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
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
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	1	1
Fair Value, Inputs, Level 3 [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	228	(186)
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	(148)	258
Fair Value, Inputs, Level 3 [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	(128)	335
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	(91)	309
Fair Value, Inputs, Level 3 [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	(103)	(151)
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	(60)	(53)
Fair Value, Inputs, Level 3 [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
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	2	1
Fair Value, Inputs, Level 3 [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
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	1	1
Fair Value, Inputs, Level 3 [Member] | 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	229	(185)
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	(149)	257
Fair Value, Inputs, Level 3 [Member] | 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	(128)	335
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	(91)	309
Fair Value, Inputs, Level 3 [Member] | 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	(103)	(151)
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	(60)	(53)
Fair Value, Inputs, Level 3 [Member] | 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
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	$ 2	$ 1