Intangible Assets - Summary of Amortization Expense (Details) - USD ($) $ in Millions		9 Months Ended	12 Months Ended
		Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Finite-Lived Intangible Assets [Line Items]
Intangible asset amortization expense			$ 92	$ 87	$ 76
Exelon Generation Co L L C [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible asset amortization expense			83	79	69
Commonwealth Edison Co [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible asset amortization expense			7	7	7
Unamortized Energy Contracts [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible asset amortization expense	[1]		35	35	22
Unamortized Energy Contracts [Member] | Exelon Generation Co L L C [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible asset amortization expense	[1]		35	35	22
Unamortized Energy Contracts [Member] | Commonwealth Edison Co [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible asset amortization expense	[1]		0	$ 0	$ 0
Unamortized Energy Contracts [Member] | Pepco Holdings LLC [Member] | Successor [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible asset amortization expense	[1]	$ 0	$ 0

[1] Included in Operating revenues or Purchased power and fuel on the Registrants’ Consolidated Statements of Operations and Comprehensive Income