Commitments and Contingencies Commitments and Contingencies - Schedule of Future Rental Expense under Operating Leases (Details) - USD ($) $ in Millions		3 Months Ended	9 Months Ended	12 Months Ended
		Mar. 23, 2016	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating Leases Future Minimum Payments Due [Line Items]
Lease and rental expense				$ 670	$ 709	$ 777
Exelon Generation Co L L C [Member]
Operating Leases Future Minimum Payments Due [Line Items]
Lease and rental expense	[1]			558	578	667
Long Term Contract For Purchase Of Electric Power Capacity			$ 604	493	508	604
Commonwealth Edison Co [Member]
Operating Leases Future Minimum Payments Due [Line Items]
Lease and rental expense				7	9	15
PECO Energy Co [Member]
Operating Leases Future Minimum Payments Due [Line Items]
Lease and rental expense				10	9	7
Baltimore Gas and Electric Company [Member]
Operating Leases Future Minimum Payments Due [Line Items]
Lease and rental expense				35	32	22
Pepco Holdings LLC [Member] | Successor [Member]
Operating Leases Future Minimum Payments Due [Line Items]
Lease and rental expense			$ 49	48	63
Pepco Holdings LLC [Member] | Predecessor [Member]
Operating Leases Future Minimum Payments Due [Line Items]
Lease and rental expense		$ 12
Potomac Electric Power Company [Member]
Operating Leases Future Minimum Payments Due [Line Items]
Lease and rental expense				10	11	8
Delmarva Power & Light Company [Member]
Operating Leases Future Minimum Payments Due [Line Items]
Lease and rental expense				13	16	15
Atlantic City Electric Company [Member]
Operating Leases Future Minimum Payments Due [Line Items]
Lease and rental expense				$ 8	$ 14	$ 13

[1] )Includes contingent operating lease payments associated with contracted generation agreements that are not included in the minimum future operating lease payments table above. Payments made under Generation’s contracted generation lease agreements totaled $493 million, $508 million and $604 million during 2018, 2017 and 2016, respectively. Excludes contract amortization associated with purchase accounting and contract acquisition