Retirement Benefits - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 23, 2016	Mar. 31, 2015
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
Pension Obligations Valuation Adjustment			35
Pension And Other Postretirement Benefit Expense Included In Capital And Operating And Maintenance Expense		$ 7
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)
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected return on assets	7.00%
Pension Plan, Defined Benefit [Member] | Exelon Legacy Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.29%
Pension Plan, Defined Benefit [Member] | PEPCO Holdings Inc [Member] | PHI Pension and Postretirement [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected return on assets	6.50%
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected return on assets	6.71%
Discount rate	4.30%
Other Postretirement Benefit Plan, Defined Benefit [Member] | PEPCO Holdings Inc [Member] | PHI Pension and Postretirement [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected return on assets	6.75%
Discount rate	4.00%