Retirement Benefits - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Mar. 31, 2016	Sep. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Increase in regulatory assets due valuation received by Exelon for its legacy pension and other postretirement benefit obligations.	$ 27
Increase in regulatory liabilities due to updated valuation of Exelon's legacy pension and postretirement benefit obligations	3
Increase (Decrease) in Pension Plan Obligations	35
Increase (Decrease) in Postretirement Obligations	8
Defined Benefit Plan, Accumulated Other Comprehensive Income Net Gains (Losses), after Tax	$ (2)
PHI Pension and Postretirement [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase (Decrease) in Pension Plan Obligations		$ 25
Pension Plan, Defined Benefit [Member] | Exelon Legacy Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected return on assets		7.00%
Discount rate		4.29%
Pension Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | PHI Pension and Postretirement [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected return on assets		6.50%
Discount rate		3.96%
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected return on assets		6.71%
Discount rate		4.29%
Other Postretirement Benefit Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | PHI Pension and Postretirement [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected return on assets		6.75%
Discount rate		3.80%