Fair Value of Financial Assets and Liabilities - Fair Value Assets and Liabilities Measure on Recurring Basis Gain Loss Included in Earnings (Details) - Fair Value, Inputs, Level 3 [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in net income	$ (23)	$ (101)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	359	160
Operating Revenue [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in net income	(219)	7
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	546	144
Purchased Power And Fuel [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in net income	245	93
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	(195)	21
Operating and Maintenance [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in net income	(3)	(4)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	3	0
Other, net [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in net income	(5)	(3)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	5	(2)
Exelon Generation Co L L C [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in net income	(26)	(105)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	356	160
Exelon Generation Co L L C [Member] | Operating Revenue [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in net income	(219)	7
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	546	144
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]
Included in net income	245	93
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	(195)	21
Exelon Generation Co L L C [Member] | Other, net [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in net income	(5)	(3)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	5	(2)
Pepco Holdings LLC [Member] | Operating and Maintenance [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in net income	(3)	(4)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	3	0
Potomac Electric Power Company [Member] | Other, net [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in net income	(3)	(4)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	3	0
6311 Life Insurance [Member] | Pepco Holdings LLC [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in net income	(3)	(4)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	3	0
6311 Life Insurance [Member] | Potomac Electric Power Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Included in net income	3	4
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	$ 3	$ 0