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, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Leases Future Minimum Payments Due [Line Items]
Lease and rental expense				$ 709	$ 777	$ 922
Exelon Generation Co L L C [Member]
Operating Leases Future Minimum Payments Due [Line Items]
Lease and rental expense	[1]			578	667	851
Long Term Contract For Purchase Of Electric Power Capacity			$ 604	508	604	798
Commonwealth Edison Co [Member]
Operating Leases Future Minimum Payments Due [Line Items]
Lease and rental expense				9	15	12
PECO Energy Co [Member]
Operating Leases Future Minimum Payments Due [Line Items]
Lease and rental expense				9	7	9
Baltimore Gas and Electric Company [Member]
Operating Leases Future Minimum Payments Due [Line Items]
Lease and rental expense				32	22	32
Pepco Holdings LLC [Member] | Successor [Member]
Operating Leases Future Minimum Payments Due [Line Items]
Lease and rental expense			$ 49	63
Pepco Holdings LLC [Member] | Predecessor [Member]
Operating Leases Future Minimum Payments Due [Line Items]
Lease and rental expense		$ 12				60
Potomac Electric Power Company [Member]
Operating Leases Future Minimum Payments Due [Line Items]
Lease and rental expense				11	8	7
Delmarva Power and Light Company [Member]
Operating Leases Future Minimum Payments Due [Line Items]
Lease and rental expense				16	15	14
Atlantic City Electric Company [Member]
Operating Leases Future Minimum Payments Due [Line Items]
Lease and rental expense				$ 14	$ 13	$ 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 $508 million, $604 million and $798 million during 2017, 2016 and 2015, respectively. Excludes contract amortization associated with purchase accounting and contract acquisitions.