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				9 Months Ended
	Sep. 30, 2018		Sep. 30, 2017		Sep. 30, 2018		Sep. 30, 2017
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				$ 3
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0				0
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	246		$ 80		172		$ 102
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	(92)		26		168		167
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	(176)		(3)		(32)		34
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	(64)		47		174		222
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	(83)		(69)		(156)		(152)
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	(40)		(23)		(15)		(61)
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	12		2		13		8
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	12		2		9		6
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				3
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0				0
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	247		81		175		104
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	(92)		25		168		165
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	(176)		(3)		(32)		34
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	(64)		47		174		222
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	(83)		(69)		(156)		(152)
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	(40)		(23)		(15)		(61)
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	12		1		13		6
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	12		1		9		4
Fair Value, Inputs, Level 3 [Member] | Pepco Holdings LLC [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			1				2
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)			1				2
Fair Value, Inputs, Level 3 [Member] | 6311 Life Insurance [Member] | Pepco Holdings LLC [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]	1	[2]	3	[1]	2	[2]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	$ 0	[1]	$ 1	[2]	$ 0	[1]	$ 2	[2]

[1] The amounts represented are life insurance contracts at Pepco.

[2] The amounts represented are life insurance contracts at Pepco.