Retirement Benefits - Balance Sheet locations of Benefit Obligations and Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Other current liabilities	$ 21	$ 21
Pension obligations	4,248	3,385
Non-pension postretirement benefit obligations	0	0
Unfunded status (net benefit obligation less plan assets)	4,269	3,406
Pension Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Predecessor [Member]
Defined Benefit Plan Disclosure [Line Items]
Other current liabilities		6
Pension obligations		466
Non-pension postretirement benefit obligations		0
Unfunded status (net benefit obligation less plan assets)		472
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Other current liabilities	31	27
Pension obligations	0	0
Non-pension postretirement benefit obligations	1,848	1,618
Unfunded status (net benefit obligation less plan assets)	$ 1,879	1,645
Other Postretirement Benefit Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Predecessor [Member]
Defined Benefit Plan Disclosure [Line Items]
Other current liabilities		0
Pension obligations		0
Non-pension postretirement benefit obligations		215
Unfunded status (net benefit obligation less plan assets)		$ 215