Related Party Transactions - Operating Revenues and Purchased Power and Fuel (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Related Party Transaction [Line Items]
Revenue from Related Parties	$ 1	$ 0
Exelon Generation Co L L C [Member]
Related Party Transaction [Line Items]
Revenue from Related Parties	330	317
Purchased power and/or fuel from affiliates	(6)	1
Exelon Generation Co L L C [Member] | Commonwealth Edison Co Affiliate [Member]
Related Party Transaction [Line Items]
Revenue from Related Parties	90	94
Purchased power and/or fuel from affiliates	7	3
Exelon Generation Co L L C [Member] | PECO Energy Co Affiliate [Member]
Related Party Transaction [Line Items]
Revenue from Related Parties	37	45
Exelon Generation Co L L C [Member] | Baltimore Gas And Electric Company Affiliate [Member]
Related Party Transaction [Line Items]
Revenue from Related Parties	99	76
Exelon Generation Co L L C [Member] | Pepco Holdings LLC Affiliate [Member]
Related Party Transaction [Line Items]
Revenue from Related Parties	103	101
Exelon Generation Co L L C [Member] | Potomac Electric Power Co Affiliate [Member]
Related Party Transaction [Line Items]
Revenue from Related Parties	79	70
Exelon Generation Co L L C [Member] | Delmarva Power and Light Co Affiliate [Member]
Related Party Transaction [Line Items]
Revenue from Related Parties	22	23
Exelon Generation Co L L C [Member] | Atlantic City Electric Co Affiliate [Member]
Related Party Transaction [Line Items]
Revenue from Related Parties	2	8
Exelon Generation Co L L C [Member] | Exelon Business Services Co Affiliate [Member]
Related Party Transaction [Line Items]
Revenue from Related Parties	1	1
Commonwealth Edison Co [Member]
Related Party Transaction [Line Items]
Revenue from Related Parties	5	4
Purchased power and/or fuel from affiliates	97	97
Commonwealth Edison Co [Member] | Exelon Generation Co LLC Affiliate [Member]
Related Party Transaction [Line Items]
Purchased power and/or fuel from affiliates	$ 97	$ 97