Related Party Transactions - Operating Revenues and Purchased Power and Fuel (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Related Party Transaction [Line Items]
Revenue from Related Parties	$ 0	$ 1	$ 0	$ 4
Exelon Generation Co L L C [Member]
Related Party Transaction [Line Items]
Revenue from Related Parties	324	331	872	932
Exelon Generation Co L L C [Member] | Commonwealth Edison Co Affiliate [Member]
Related Party Transaction [Line Items]
Revenue from Related Parties	96	71	249	241
Purchased power and/or fuel from affiliates	2	0	7	11
Exelon Generation Co L L C [Member] | PECO Energy Co Affiliate [Member]
Related Party Transaction [Line Items]
Revenue from Related Parties	59	68	142	146
Exelon Generation Co L L C [Member] | Baltimore Gas And Electric Company Affiliate [Member]
Related Party Transaction [Line Items]
Revenue from Related Parties	65	84	195	252
Exelon Generation Co L L C [Member] | Pepco Holdings LLC Affiliate [Member]
Related Party Transaction [Line Items]
Revenue from Related Parties	99	105	276	288
Exelon Generation Co L L C [Member] | Potomac Electric Power Co Affiliate [Member]
Related Party Transaction [Line Items]
Revenue from Related Parties	69	80	199	219
Exelon Generation Co L L C [Member] | Delmarva Power and Light Co Affiliate [Member]
Related Party Transaction [Line Items]
Revenue from Related Parties	25	21	63	60
Exelon Generation Co L L C [Member] | Atlantic City Electric Co Affiliate [Member]
Related Party Transaction [Line Items]
Revenue from Related Parties	5	4	14	9
Exelon Generation Co L L C [Member] | Exelon Business Services Co Affiliate [Member]
Related Party Transaction [Line Items]
Revenue from Related Parties	5	3	10	5
Commonwealth Edison Co [Member]
Related Party Transaction [Line Items]
Revenue from Related Parties	9	15	19	31
Purchased power and/or fuel from affiliates	93	71	256	252
Commonwealth Edison Co [Member] | Exelon Generation Co LLC Affiliate [Member]
Related Party Transaction [Line Items]
Purchased power and/or fuel from affiliates	$ 94	$ 71	$ 256	$ 252