Lessee - Supplemental Cash Flow Information Related to Leases (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Supplemental Cash Flow Information [Line Items]
Operating cash flows from operating leases	$ 271	$ 287
Right-of-Use Asset Obtained in Exchange for Operating Lease Liability	1	52
Exelon Generation Co L L C [Member]
Supplemental Cash Flow Information [Line Items]
Operating cash flows from operating leases	204	206
Right-of-Use Asset Obtained in Exchange for Operating Lease Liability	3	14
Commonwealth Edison Co [Member]
Supplemental Cash Flow Information [Line Items]
Operating cash flows from operating leases	3	3
Right-of-Use Asset Obtained in Exchange for Operating Lease Liability	0	6
PECO Energy Co [Member]
Supplemental Cash Flow Information [Line Items]
Operating cash flows from operating leases	1	0
Right-of-Use Asset Obtained in Exchange for Operating Lease Liability	1	0
Baltimore Gas and Electric Company [Member]
Supplemental Cash Flow Information [Line Items]
Operating cash flows from operating leases	20	33
Right-of-Use Asset Obtained in Exchange for Operating Lease Liability	0	2
Pepco Holdings LLC [Member]
Supplemental Cash Flow Information [Line Items]
Operating cash flows from operating leases	39	37
Right-of-Use Asset Obtained in Exchange for Operating Lease Liability	(1)	(3)
Finance Lease, Principal Payments	6
Right-of-Use Asset Obtained in Exchange for Finance Lease Liability	29
Potomac Electric Power Company [Member]
Supplemental Cash Flow Information [Line Items]
Operating cash flows from operating leases	8	9
Right-of-Use Asset Obtained in Exchange for Operating Lease Liability	0	(1)
Finance Lease, Principal Payments	2
Right-of-Use Asset Obtained in Exchange for Finance Lease Liability	8
Delmarva Power & Light Company [Member]
Supplemental Cash Flow Information [Line Items]
Operating cash flows from operating leases	9	6
Right-of-Use Asset Obtained in Exchange for Operating Lease Liability	(1)	(2)
Finance Lease, Principal Payments	3
Right-of-Use Asset Obtained in Exchange for Finance Lease Liability	14
Atlantic City Electric Company [Member]
Supplemental Cash Flow Information [Line Items]
Operating cash flows from operating leases	4	5
Right-of-Use Asset Obtained in Exchange for Operating Lease Liability	0	$ (1)
Finance Lease, Principal Payments	1
Right-of-Use Asset Obtained in Exchange for Finance Lease Liability	$ 7