Fair Value of Financial Assets and Liabilities Fair Value of Financial Assets and Liabilities - Fair Value Assets and Liabilities Measure on Recurring Basis Gain Loss Included in Earnings (Details) - USD ($) $ in Millions		3 Months Ended	9 Months Ended	12 Months Ended
		Mar. 23, 2016	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016
Operating Revenue [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings				$ 28	$ (477)
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)				290	154
Purchased Power And Fuel [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings				(126)	(91)
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)				(36)	(45)
Other, net [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	[1]			6	10
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	[1]			4	7
Operating and Maintenance [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings				3
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)				3
Exelon Generation Co L L C [Member] | Operating Revenue [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings				28	(477)
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)				290	154
Exelon Generation Co L L C [Member] | Purchased Power And Fuel [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings				(126)	(91)
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)				(36)	(45)
Exelon Generation Co L L C [Member] | Other, net [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	[1]			6	7
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	[1]			4	5
Pepco Holdings LLC [Member] | Other, net [Member] | Successor [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	[1],[2]		$ 3
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	[1],[2]		$ 2
Pepco Holdings LLC [Member] | Other, net [Member] | Predecessor [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [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
Pepco Holdings LLC [Member] | Operating and Maintenance [Member] | Successor [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings				3
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)				3
Potomac Electric Power Company [Member] | Other, net [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	[1]			0	3
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	[1]			0	3
Potomac Electric Power Company [Member] | Operating and Maintenance [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings				3
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)				3
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings				81	557
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)				261	116
Fair Value, Inputs, Level 3 [Member] | Exelon Generation Co L L C [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings				84	560
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)				258	114
Fair Value, Inputs, Level 3 [Member] | 6311 Life Insurance [Member] | Pepco Holdings LLC [Member] | Successor [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings				(3)	(3)	[3]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)				3	2	[3]
Fair Value, Inputs, Level 3 [Member] | 6311 Life Insurance [Member] | Pepco Holdings LLC [Member] | Predecessor [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [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
Fair Value, Inputs, Level 3 [Member] | 6311 Life Insurance [Member] | Potomac Electric Power Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings				(3)	(3)
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)				$ 3	$ 3
Fair Value, Inputs, Level 3 [Member] | Preferred Stock [Member] | Pepco Holdings LLC [Member] | Predecessor [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [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 to December 31, 2016. See the subsequent table for PHI's predecessor periods, as well as activity for Pepco for the year ended December 31, 2017 and 2016.

[3] Successor period represents activity from March 24, 2016 to December 31, 2016. See tables below for PHI's predecessor periods, as well as activity for Pepco for the years ended December 31, 2017 and 2016.