Related Party Transactions - Operating Revenues and Purchased Power and Fuel (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Related Party Transaction [Line Items]
Revenue from Related Parties	$ 1	$ 0	$ 2	$ 0
Exelon Generation Co L L C [Member]
Related Party Transaction [Line Items]
Revenue from Related Parties	271	252	601	569
Purchased power and/or fuel from affiliates	(3)	3	(9)	4
Exelon Generation Co L L C [Member] | Commonwealth Edison Co Affiliate [Member]
Related Party Transaction [Line Items]
Revenue from Related Parties	80	89	170	183
Purchased power and/or fuel from affiliates	4	1	11	4
Exelon Generation Co L L C [Member] | PECO Energy Co Affiliate [Member]
Related Party Transaction [Line Items]
Revenue from Related Parties	41	35	78	80
Exelon Generation Co L L C [Member] | Baltimore Gas And Electric Company Affiliate [Member]
Related Party Transaction [Line Items]
Revenue from Related Parties	70	57	169	133
Exelon Generation Co L L C [Member] | Pepco Holdings LLC Affiliate [Member]
Related Party Transaction [Line Items]
Revenue from Related Parties	78	69	182	170
Exelon Generation Co L L C [Member] | Potomac Electric Power Co Affiliate [Member]
Related Party Transaction [Line Items]
Revenue from Related Parties	60	52	139	122
Exelon Generation Co L L C [Member] | Delmarva Power and Light Co Affiliate [Member]
Related Party Transaction [Line Items]
Revenue from Related Parties	16	12	38	35
Exelon Generation Co L L C [Member] | Atlantic City Electric Co Affiliate [Member]
Related Party Transaction [Line Items]
Revenue from Related Parties	2	5	5	13
Exelon Generation Co L L C [Member] | Exelon Business Services Co Affiliate [Member]
Related Party Transaction [Line Items]
Revenue from Related Parties	2	2	2	3
Commonwealth Edison Co [Member]
Related Party Transaction [Line Items]
Revenue from Related Parties	11	5	16	9
Purchased power and/or fuel from affiliates	84	91	181	187
Commonwealth Edison Co [Member] | Exelon Generation Co LLC Affiliate [Member]
Related Party Transaction [Line Items]
Purchased power and/or fuel from affiliates	$ 84	$ 90	$ 181	$ 187