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		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Mar. 23, 2016	Sep. 30, 2016	[2]	Sep. 30, 2017	Sep. 30, 2016
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		$ 80	$ (93)				$ 102	$ 334
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		26	288				167	248
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		(3)	180				34	(232)
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		47	323				222	303
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		(69)	(85)				(152)	(107)
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		(23)	(38)				(61)	(63)
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	(3)				8	4
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	[1]	2	3				6	8
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		81	(92)				104	336
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		25	288				165	247
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		(3)	180				34	(232)
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		47	323				222	303
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		(69)	(85)				(152)	(107)
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		(23)	(38)				(61)	(63)
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	[1]	1	(4)				6	2
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	[1]	1	3				4	7
Pepco Holdings LLC [Member] | Other, net [Member] | Successor [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	2
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	[1]	1	0				2	1
Pepco Holdings LLC [Member] | Other, net [Member] | Predecessor [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]			$ (17)
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	[1]			1
Potomac Electric Power Company [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]	1	1				2	3
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	[1]	1	0				2	2
6311 Life Insurance [Member] | Pepco Holdings LLC [Member] | Successor [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, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		1	0		$ 1		2
6311 Life Insurance [Member] | Pepco Holdings LLC [Member] | Predecessor [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)
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)				1
6311 Life Insurance [Member] | Potomac Electric Power Company [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)	(3)
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		$ 1	$ 0				$ 2	$ 2
Preferred Stock [Member] | Pepco Holdings LLC [Member] | Predecessor [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				18
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)				$ 0

[1] Other, net activity consists of realized and unrealized gains (losses) included in income for the NDT funds held by Generation, accrued interest on a convertible promissory note at Generation and the life insurance contracts held by PHI and Pepco.

[2] Successor period represents activity from March 24, 2016 through September 30, 2016. See tables below for PHI's predecessor periods, as well as activity for Pepco for the three and nine months ended September 30, 2017 and 2016.