Retirement Benefits Retirement Benefits - Components of Accumulated Other Comprehensive Income and Regulatory Assets (Liabilities) related to Retirement Plans (Details) - USD ($) $ in Millions		3 Months Ended		12 Months Ended
		Mar. 31, 2018	Mar. 23, 2016	Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, amounts recognized in regulatory assets, before tax		$ 61
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Current year actuarial (gain) loss				$ 635	$ (222)	[1]	$ 644
Amortization of actuarial loss				(629)	(607)	[1]	(554)
Current year prior service cost (credit)				(4)	9	[1]	(60)
Amortization of prior service (cost) credit				(2)	(1)	[1]	(14)
Settlements				3	(3)	[1]	0
DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeAndRegulatoryAssetsAcqusition				0	0	[1]	994
Total recognized in AOCI and regulatory assets (liabilities)	[1]			(3)	(824)		1,010
Defined benefit plan, amounts recognized in OCI, before tax				3	(401)	[1]	51
Defined benefit plan, amounts recognized in regulatory assets, before tax				(6)	(423)	[1]	959
Pension Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Predecessor [Member]
Defined Benefit Plan Disclosure [Line Items]
Current year actuarial (gain) loss			$ 0
Amortization of actuarial loss			(14)
Amortization of prior service (cost) credit			0
Total recognized in AOCI and regulatory assets (liabilities)	[1]		(14)
Defined benefit plan, amounts recognized in OCI, before tax			(1)
Defined benefit plan, amounts recognized in regulatory assets, before tax			(13)
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Current year actuarial (gain) loss				(232)	166	[1]	(101)
Amortization of actuarial loss				(66)	(61)	[1]	(63)
Current year prior service cost (credit)				0	0	[1]	0
Amortization of prior service (cost) credit				186	188	[1]	185
Settlements				0	0	[1]	0
DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeAndRegulatoryAssetsAcqusition				0	0	[1]	94
Total recognized in AOCI and regulatory assets (liabilities)	[1]			(112)	293		115
Defined benefit plan, amounts recognized in OCI, before tax				(55)	168	[1]	20
Defined benefit plan, amounts recognized in regulatory assets, before tax				$ (57)	$ 125	[1]	$ 95
Other Postretirement Benefit Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Predecessor [Member]
Defined Benefit Plan Disclosure [Line Items]
Current year actuarial (gain) loss			0
Amortization of actuarial loss			(2)
Amortization of prior service (cost) credit			3
Total recognized in AOCI and regulatory assets (liabilities)	[1]		1
Defined benefit plan, amounts recognized in OCI, before tax			0
Defined benefit plan, amounts recognized in regulatory assets, before tax			$ 1

[1] 2016 amounts include PHI for the period of March 24, 2016 through December 31, 2016.